In poetry and fiction, the main reason for using vivid words is to
a. keep the reader guessing.
b. explain an idea.
c. illustrate the writer's bias.
d. create a mental image.

The correct answer is D. Create a mental image. It is important for the reader to be able to image the situations. This is why good writers use vivid words that can create a mental image that a reader can understand. There have even been, throughout history, entire literature movements based on writing using vivid words to describe the world, including even the minutest details.

its c