Antecedents should be placed <u><em>before</em></u> the pronouns they modify.
This seems kind of opinionated; however, I would go with the most vague, least specific question, because one could probably write more about that topic. So, "How many teens participate in extracurricular activities every year?" it is!
The best resource for comparing and contrasting information on cancer diseases would be option C --The science of malignant tumors. Option A is about how to beat a specific type of cancer but it does not compare or contrast information on different types of cancer. Option B seems to be a kind of biography, which may or may not contain specific information about cancer. Option D is a medical dictionary which probably contains information about different kinds of cancer, but to use it, the reader should know what word to look up. Option C is the only one addressing malignant tumors, and therefore, it is to be expected to provide information to compare and contrast different types of cancer.
