The brainstorming technique in which you spend a predetermined period of time writing nonstop about a specific topic, and then select a sentence or phrase to start a new free-writing session is called

The brainstorming technique in which you spend a predetermined period of time writing nonstop about a specific topic, and then select a sentence or phrase to start a new free-writing session is called looping.

Explanation:

Looping is a technique in which free-writing is used to help narrow a topic. You begin by writing freely about something for a certain period of time. Then, you choose one of the ideas, sentences, or words that you have written and begin to write freely again, having it as your topic. You can repeat the process once more, which will help you narrow it down even more.

a, b, and c are all correct.