C. If you really stop and think about it, C is the most definite answer.
This question is called analogy, the comparison of two things or how they are related, im not sure with my answer but im really want to help you though so here are my answers:
21. C. provocative: dull
22. A. scholar: encyclopedia
23. D. literal:figurative
24. A. olfactory:sight
25. B. re: again