Hi, I believe this solution to the challenge works better so you don’t need the int correctAnswerIndex; variable. Let me know if you see any glaring issues. 
Here is it working in Unity:
Here is the full code:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using TMPro;
using UnityEngine.UI;
public class Quiz : MonoBehaviour
{
[SerializeField] QuestionSO question;
[SerializeField] TextMeshProUGUI questionText;
[SerializeField] GameObject[] answerButtons;
//int correctAnswerIndex; Not needed with my solution
[SerializeField] Sprite defaultAnswerSprite;
[SerializeField] Sprite correctAnswerSprite;
void Start()
{
questionText.text = question.GetQuestion();
for (int i = 0;i<answerButtons.Length;i++)
{
TextMeshProUGUI buttonText = answerButtons[i].GetComponentInChildren<TextMeshProUGUI>();
buttonText.text = question.GetAnswer(i);
}
}
public void OnAnswerSelected(int index)
{
Image buttonImage;
if (index == question.GetCorrectAnswerIndex())
{
questionText.text = "Correct!";
buttonImage = answerButtons[index].GetComponent<Image>();
buttonImage.sprite = correctAnswerSprite;
}
else
{
//Sets question text to the correct answers using GetAnswer method with GetCorrectAnswerIndex method as variable.
questionText.text = "The answer was:\n" + question.GetAnswer(question.GetCorrectAnswerIndex());
//Sets the button's image using same GetCorrectAnswerIndex method as variable.
buttonImage = answerButtons[question.GetCorrectAnswerIndex()].GetComponent<Image>();
buttonImage.sprite = correctAnswerSprite;
}
}
}