Asking the question why is this true? is a good way to generate
a. supporting materials
b. a thesis statement
c. main ideas
d. a conclusion

The answer is C. main idea

a. supporting materials

Explanation:

Support material is all that part of your text or speech that backs up your claim or main argument. By asking why this is true and knowing how to answer it, you know how to explain to the audience or readers why your claim is true. You need to share reliable quality information that you have gathered to convince the audience.

There was a beautiful rainbow.

It's Simple