Which sentence best describes clustering?
a. you're generating words that suggest possible sentences or paragraphs.
b. you're generating words that suggest possible themes for an essay.
c. you write down words or ideas in chronological order.
d. you write down words or ideas that occur to you in no particular order.

The answer is letter A. You're generating words that suggest possible sentences or paragraphs.
It is not chronological or suggests possible themes. Clustering only happens when there is a theme already. When in clustering, thoughts are tumbled out, enlarging vocabulary word bank and enabling to see patterns of ideas. It may be done individually or by class.
Clustering is akin to brainstorming. It is an activity wherein ideas, images, and feelings are generated around a stimulus. 

Are those the answer choices ?