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.
Hope that helpful --------------------------------------
The answer is personal and spiritual exploration because romantic literature was meant to make the mind wander into romantic stories of love and romance where one found out more about themselves while falling in love. At least, that is my best guess since it's certainly not supposed to be rational and well reasoned ideas, it's not about religion, and I don't think it's supposed to empasise on society.