The answer should be quite obvious. They gave you the answer in the question, 5^3/2. All the other answers result in 5 cubed.
Now, for actually determining it when not given choices. The nth root, or just the number on the outside, is always the denominator in the fractional power. In a regular square root, the n is just 2. In this case, the n is equal to 2. So you are taking the 2nd root (squard root) of the number. That means the denominator is 100% going to be 2. the numerator is simply the power that the number inside is going to. In this case it is 3.
