Question

Good writers:

a. choose diction to go with purpose and audience
b. show off with formal diction
c. always simplify their diction
d. develop one style of diction and stick with it

I think its C.

Answer 1

I think it's a.
After all, sometimes for descriptive passages you want elaborate diction, so C wouldn't work.
And if you're writing a book for kids, you need to choose diction that kids will understand; if you're writing a textbook for doctors, you should use very different diction.
So a seems correct to me.