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.

faulty parallelism is a construction in which two or more parts of a sentence are equivalent in meaning but not grammatically similar in form.

the correct version of this faulty parallelism in the sentence is:

the hikers knew that the hike was six miles, that it usually required the entire day, and it would not be wise to start after 10: 00 a.m.

the problem is the last sentence, the first two have a basic subject+verb+object structure.

so, to give a proper balance to the structure, or better said, to give a parallel structure, we must write a basic s+v+o structure in all three sentences:

the hikers knew that the hike was six miles, that it usually required the entire day, and it would not be wise to start after 10: 00 a.m.

not sure how to answer your question sorry