It would be D.
First of all, consider one side of a cube as x. If x is raised to the third, it would equal the area of that cube, maybe y. For example, if the side of a cube was 10 m and you had to find the volume of that cube, 10^3= 10x10x10= 1000 m^3.
For this problem, you just have to reverse the process. Instead of cubing a side to find the volume, take the third root of the volume to get the side length. So, if y is the volume of a cube, take the third root of y which could equal x. If you were to relate it the previous example, 1000 is the volume. So, ³√1000= 10.
Therefore, to find the side length of a cube with volume 125m^3, you would have to ³√125m = 5.