Synonyms can be used in a text in order to:
a. add variety to word choice.
b. add emotional appeal.
c. confuse the reader.
d. create internal rhymes.

The correct answer is A, to add variety to word choice. Synonyms are words which have the same meaning, so in order not to repeat the same word over and over again, you just use a word which has the same meaning, but has a different form, so that you can add some variety.