The surface area of a cube can be found by the formula 6*a^2, where `a` is the length of its sides. If the surface area is 216 sq.m, we use the formula to find the length of the sides, i.e., a = sqrt(216/6) = 6m.

Option 1 (100 cu m) is incorrect because the volume of a cube is calculated by the formula a^3. Therefore, if `a` is 6m, then the volume is 6^3 = 216 cu m, not 100 cu m.

Option 2 (216 cu m) is the correct answer. As explained above, if `a` is 6m, then the volume is 6^3 = 216 cu m.

Option 3 (480 cu m) is incorrect. Using the correct formula, the volume is 6^3 = 216 cu m, not 480 cu m.

Option 4 (512 cu m) is incorrect. Using the correct formula, the volume is 6^3 = 216 cu m, not 512 cu m.

In conclusion, the volume of a cube with a surface area of 216 sq.m is indeed 216