How does identifying the main idea help to determine the author's purpose on a topic?

it helps because you can determine the writers perspective which will lead to the purpose.

Explanation:

The main idea of a paragraph is the primary point or concept that the author wants to communicate to the readers about the topic. in a paragraph, when the main idea is stated directly, it is expressed in what is called the topic sentence.

i would pick a. i think this is more of a personal question though.