Final answer:
To express the volume of a cube with an edge length of 21 cm using an exponent, the correct answer is 21^3.
Step-by-step explanation:
To express the volume of a cube with an edge length of 21 cm using an exponent, we can use the formula V = s^3, where V represents the volume and s represents the length of one side.
Plugging in the value for s, which is 21 cm, we get V = 21^3.
Therefore, the correct answer is b. 21^3 as it uses an exponent to express the volume of the cube.