My answer may be wrong or it may be right but i think it is C
A is the most suitable answer, although all of the answers are technically correct. B, C and D all serve to add <em>clarity</em> to an essay, which in turn adds interest. A directly adds interest, and the clarity comes as a secondary effect.
Girl i have no idea... but go to oogle on ur phone and look up example of linking verbs that should help
