Since right triangles all follow the Pythagorean Theorem (a^2 + b^2 = c^2), you would use the Pythagorean Theorem to figure this out:
A). 6^2 + 7^2 =? 13^2
36 + 49 =? 169
85 is not equal to 169.
B). (sqrt (21))^2 + (sqrt (99))^2 =? 11^2
21 + 99 =? 121
120 is not equal to 121.
C). 10^2 + 60^2 =? 61^2
100 + 3600 =? 3721
3700 is not equal to 3721.
D). (sqrt (35))^2 + (sqrt (14))^2 =? 7^2
35 + 14 =? 49
49 is equal to 49.
The answer is (D).