Let's take a look at the exponent rule:

Her mistake was she multiplied the exponents instead of adding it up.
Step-by-step explanation:
since all sides of a cube have the same length (same principle as e.g. for a square), this means also the length and width are 10 in long.
the volume is therefore
10 × 10 × 10 = 1000 in³
I think it's the third choice :)) the one you currently have