Sentence syntax should be varied to

A. avoid unexpected words.

B. summarize main ideas.

C. keep readers' interest.

D. provide supporting details.

C. To keep the reader's interest. Syntax is the structure of a sentence (how long it is and how it is punctuated). It does not really deal with the specific, individual words that an author chooses--so a, b and c would not be the answer because they deal with the actual words in the text.