Using synonyms can make your writing
a. understanding.
b. boring.
c. interesting.
d. common

c. interesting.

Explanation:

The use of synonyms can help you make your reading more interesting because it gives yout audience the opportunity to learn and to read new different words that weren´t in their vocabulary prior to reading your texts, using the same words over and over is tiring for those who read you, that´s why every writer needs to have a wide lexic and know a lot of synonyms to express widely different ideas.

i’m pretty sure it is the second