The correct answer to this question is A, 5. This type of question would be found on a test for early learners, such as kindergartners, who are just starting to learn numbers and the order of those numbers. To properly answer this question, the student would need to be able to at least count up to the number 6.
If they can, they would know the order is 1, 2, 3, 4, 5, 6. Knowing that the student can focus on numbers 4 through 6. From the question, they would know there is only one number between those two, and that number is 5.