Evaluate the integral: ∫x³√(16 - x²) dx + C.
a) (1/5) (16 - x²) (3/2) + C
b) (1/5) (16 - x²) (5/2) + C
c) (1/5) (16 - x²) (2/3) + C
d) (1/5) (16 - x²) (4/3) + C