We have to find f(x).
First we have to integrate f'(x) and then satisfy the initial condition f(0).
Integrating f'(x):
We can then replace x with x=0 and f(x) with f(0)=21 to find the value of the constant C:
Then, the function f(x) is:
Answer: f(x) = x^3/3 + 3x + 21 (Option A)