The best answer is A, a map. The reason why is because since the student's purpose is to inform others about the great distances caribou migrate just to obtain food, a map showing how far the caribou migrates is going to best help the viewers understand the student.
C won't work because the student's purpose and the food sources doesn't relate to one another.
D won't work because once again, a photograph of caribou eating isn't going to help show viewers how caribou migrate great distances to obtain food.