The best answer is C.
Option C acknowledges the student has contradicted herself without being judgmental. It also allows for further inspection and discussion by having everyone think about the research and decide which contradictory opinion is the best one.
Option A totally changes the subject, which is not the best way to resolve the contradiction. Option B tells the student what to think instead of allowing her to arrive at her own conclusion. Option D does not bring the contradiction to the student's awareness.
Therefore, Option C is the best way to resolve the contradiction.