My solution for Challenge number two: Tracking Cheat Status by Question

First I removed the var isCheater from QuizViewModel.kt, instead of using it, I decided to use the structure very similar from Challenge from Chapter 3, and using the wasAnswered variable from Question.kt, and mark each question cheated (marked as true). So, knowing the user cheated in some specific question, in this way in onActivityForResult in MainActivity.kt:

quizViewModel.questionBank[quizViewModel.currentIndex].wasAnswered =
                data?.getBooleanExtra(EXTRA_ANSWER_SHOWN, false) ?: false

The final Result is:
MainActivity.kt

private const val TAG = "MainActivity"
private const val BUNDLE_KEY_INDEX = "index"
private const val REQUEST_CODE_CHEAT = 0

class MainActivity : AppCompatActivity() {

    private lateinit var trueButton: Button
    private lateinit var falseButton: Button
    private lateinit var cheatButton: Button
    private lateinit var nextButton: Button
    private lateinit var questionTextView: TextView

    private val quizViewModel: QuizViewModel by lazy {
        ViewModelProviders.of(this).get(QuizViewModel::class.java)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        Log.d(TAG, "onCreate(Bundle?) called")

        setContentView(R.layout.activity_main)

        val currentIndex = savedInstanceState?.getInt(BUNDLE_KEY_INDEX, 0) ?: 0
        quizViewModel.currentIndex = currentIndex

        trueButton = findViewById(R.id.true_button)
        falseButton = findViewById(R.id.false_button)
        cheatButton = findViewById(R.id.cheat_button)
        nextButton = findViewById(R.id.next_button)

        questionTextView = findViewById(R.id.question_text_view);

        trueButton.setOnClickListener { view: View ->
            checkAnswer(true)
        }

        falseButton.setOnClickListener { view: View ->
            checkAnswer(false)
        }

        nextButton.setOnClickListener { view: View ->
            quizViewModel.moveToNext()
            updateQuestion()
        }

        cheatButton.setOnClickListener { view: View ->
            val answerIsTrue = quizViewModel.currentQuestionAnswer
            val intent = CheatActivity.newIntent(this@MainActivity, answerIsTrue)
            startActivityForResult(intent, REQUEST_CODE_CHEAT)
        }

        updateQuestion()
    }

    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)

        if (resultCode != Activity.RESULT_OK) {
            return
        }

        if (requestCode == REQUEST_CODE_CHEAT) {
            quizViewModel.questionBank[quizViewModel.currentIndex].wasAnswered =
                data?.getBooleanExtra(EXTRA_ANSWER_SHOWN, false) ?: false
        }
    }

    override fun onStart() {
        super.onStart()
        Log.d(TAG, "onStart() called")
    }
    override fun onResume() {
        super.onResume()
        Log.d(TAG, "onResume() called")
    }
    override fun onPause() {
        super.onPause()
        Log.d(TAG, "onPause() called")
    }

    override fun onSaveInstanceState(savedInstanceState: Bundle) {
        super.onSaveInstanceState(savedInstanceState)
        savedInstanceState.putInt(BUNDLE_KEY_INDEX, quizViewModel.currentIndex)
    }

    override fun onStop() {
        super.onStop()
        Log.d(TAG, "onStop() called")
    }
    override fun onDestroy() {
        super.onDestroy()
        Log.d(TAG, "onDestroy() called")
    }

    private fun updateQuestion() {
        Log.d(TAG, "Updating question text", Exception())
        val questionTextResId = quizViewModel.currentQuestionText
        questionTextView.setText(questionTextResId)
    }

    private fun checkAnswer(userAnswer: Boolean) {
        val correctAnswer = quizViewModel.currentQuestionAnswer

        val messageResId = when {
            quizViewModel.questionBank[quizViewModel.currentIndex].wasAnswered -> R.string.judgment_toast
            userAnswer == correctAnswer -> R.string.correct_toast
            else -> R.string.incorrect_toast
        }
        Toast.makeText(this, messageResId, Toast.LENGTH_SHORT).show()
    }

CheatActivity.kt

const val EXTRA_ANSWER_SHOWN = "com.rpizzolato.geoquiz.answer_shown"
private const val EXTRA_ANSWER_IS_TRUE = "com.rpizzolato.geoquiz.answer_is_true"
private const val KEY_WAS_CHEATED = "was_cheated"

class CheatActivity : AppCompatActivity() {
    private lateinit var answerTextView: TextView
    private lateinit var showAnswerButton: Button
    private var answerIsTrue = false
    private var wasCheated = false

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_cheat)

        wasCheated = savedInstanceState?.getBoolean(KEY_WAS_CHEATED, false) ?: false
        setAnswerShownResult(wasCheated)

        answerIsTrue = intent.getBooleanExtra(EXTRA_ANSWER_IS_TRUE, false)

        answerTextView = findViewById(R.id.answer_text_view)
        showAnswerButton = findViewById(R.id.show_answer_button)

        showAnswerButton.setOnClickListener {
            wasCheated = true
            setAnswerShownResult(true)
            fillTextIfCheated()
        }

        fillTextIfCheated()
    }

    private fun fillTextIfCheated() {
        if (wasCheated) {
            val answerText = when {
                answerIsTrue -> R.string.true_button
                else -> R.string.false_button
            }
            answerTextView.setText(answerText)
        }
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putBoolean(KEY_WAS_CHEATED, wasCheated)
    }

    private fun setAnswerShownResult(isAnswerShown: Boolean) {
        if (wasCheated) {
            val data = Intent().apply {
                putExtra(EXTRA_ANSWER_SHOWN, isAnswerShown)
            }
            setResult(Activity.RESULT_OK, data)
        }
    }

    companion object {
        fun newIntent(packageContext: Context, answerIsTrue: Boolean): Intent {
            return Intent(packageContext, CheatActivity::class.java).apply {
                putExtra(EXTRA_ANSWER_IS_TRUE, answerIsTrue)
            }
        }
    }
}

QuizViewModel.kt

private const val TAG = "QuizViewModel"

class QuizViewModel: ViewModel() {

    val questionBank = listOf(
        Question(R.string.question_australia, true),
        Question(R.string.question_oceans, true),
        Question(R.string.question_mideast, false),
        Question(R.string.question_americas, false),
        Question(R.string.question_asia, true))

    var currentIndex = 0

    val currentQuestionAnswer: Boolean get() = questionBank[currentIndex].answer
    val currentQuestionText: Int get() = questionBank[currentIndex].textResId

    fun moveToNext() {
        currentIndex = (currentIndex + 1) % questionBank.size
    }
}

Question.kt

data class Question (@StringRes
                     val textResId: Int,
                     val answer: Boolean,
                     var wasAnswered: Boolean = false)

Please, feel free to comment this solution, I appreciate it.