The most probable and most likely answer to this problem would be C. revealed much about their behavior.
Out of the four options presented in this problem, C. would only need a complete subject to make it a correct sentence. Options A. by closely watching and gathering data and B. to terrifying effect would probably need other more words and/or phrases to make them more complete. D. insect studies, on the other hand, can also be considered as a correct option IF insect become the subject.
I think that right annswer for this question is a.
13-6=7 or
13-7=6!
I hope this helps!
