Answer: C) 3, 4, 5
You use the pythagorean theorem to check
Square the smaller sides 3 and 4, then add those results up
3^2 = 9
4^2 = 16
9+16 = 25
Note how this is equal to the value of 5^2 which is squaring the longest side of the 3-4-5 triangle.
Another way you can write it down is like this
a^2 + b^2 = c^2
3^2 + 4^2 = 5^2
9 + 16 = 25
25 = 25