If you have a lot of Pythagorean triples memorized, then you'd probably know immediately, but if not, then you can use the Pythagorean Theorem to calculate each one:
A.: 7^2+9^2=49+81 which is not equal to 13^2, or 169
B.: 5^2+12^2=25+144 which is not equal to 14^2, or 196
C.: 8^2+15^2=64+225=17^2=289
D.: 9^2+12^2=81+144 which isn't equal to 14^2, or 196
Therefore, the correct answer is C.
