Answer:
A. 15
Explanation:
Ooh, look! This triangle is a Pythagorean triple! Without even needing to use a formula, I can tell that you are correct and the answer is A. 15.
It's based off of a 3-4-5 right triangle, because 9-12-15 simplifies to that if you divide every term by 3. It's basically just a bigger version of a 3-4-5 triangle! Isn't math cool:)