Online test with 80 questions best practices

I have to create an online test that has around 80 questions. These are one sentence questions with 5 answer options for all questions (no free form). Now my question is what would be the optimal way of displaying the question for a UX point of view.

Either showing 20 questions per page or showing one question per page and when an answer is selected it automatically and in a fast way takes the user to the new question (allowing user to go back if the selected answer is incorrect).