Which one of the following sets of numbers represents the lengths of the sides of a right triangle ?

a. 13 35 37
b. 6 9 10
c. 5 12 13
d. 9 40 42

Let "a", "b" and "с"  be sides of the triangle ("с" is the longest side). The triangle will be right if a² + b² = c²

a. 13 35 37
13² + 35² = 1394  and 37² = 1369
1394 ≠ 369

b. 6 9 10 
6² + 9² = 117  and 10² = 100
117 ≠ 100

c. 5 12 13
5² + 12² = 169  and 13² = 169
169 = 169   ←    right triangle 

d. 9 40 42
9² + 40² = 1681  and 42² = 1764
1681 ≠ 1784

The answer is c. 5 12 13