1. What's your favorite class?

Question

2. It's math.
3. It's math class.
4. It's a math class.
5. My favorite class is math.

Which are the right answers for the question?

6. What's your favorite subject?
7. It's math.

(What about the question, #6? Is the question the same as question 1 or different one?)

Answer 1

All of the answers are correct.

Numbers 1 and 6 are nearly the same. Number 1 implies a class you're taking now. Number 6 could be any class -- one you're taking now or one you took a couple of years ago.