Final answer:
The length of a side of the cube is 9 cm.
Step-by-step explanation:
To find the length of a side of a cube, you need to find the cube root of its volume. In this case, the volume of the cube is given as 729 cm³. Taking the cube root of 729 gives us the length of one side, which is 9 cm. Therefore, the correct answer is s = 9 cm.