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.
Sample response: The general lesson from the reading is that inventing a useful product takes time and perseverance. You have to experiment, fail, and retry your ideas until you succeed.
