The correct answer is THE answer that makes you feel best about your self, so put what makes you the happiest on the page and the teacher will give you a gold star. And we all know that life is not about learning it is about collecting gold stars.
Sorry, I haven't finished my coffee yet. I believe the answer is 12. That is the one that makes me feel the best right now. Maybe when I finish my coffee that will change.
C.G.B.

12 is the answer.
9 x 10 = 90
8 x 9 = 72
7 x 8 = 56
6 x 7 = 42
3 x 4 = 12

9 is the answer

Again, here is a test with no parameters stated. If you come up with an answer, and can show your work, and the math is done correctly, then your answer is just as valid as any other.