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 question is asking us to answer what would be generated if we ask "Why is this true". If you look at it, you will see that the question will generate a reason or multiple reasons for something - so it would likely give some support to the main point. Likely also supporting materials might be found. I think the best answer is a) . If the question was only asking "is this true" then we would choose the answer d. a conclusion