Final answer:
The expression 64x^9 can be written as a cube of a monomial by cubing the digit term and multiplying the exponent by 3. The correct answer is (4x^3)^3.
Step-by-step explanation:
The expression 64x^9 can be written as a cube of a monomial by applying the rule of cubing exponentials. To do this, we cube the digit term (64) in the usual way and multiply the exponent of the exponential term (x^9) by 3. So the correct answer is (4x^3)^3, option A.