For a conservative vector field F below, which of the following is the scalar potential?
$$F = (y^2 \cos x + az^3)\overrightarrow{i}+(2y \sin x - 4)\overrightarrow{j}+(3xz^2 + 2)\overrightarrow{k}$$ a is an integer

A

Cannot be found

B

$$xz^3 + y^2 \sin x - 4y + 2z$$

C

$$2y \cos x - y^2 \sin x - 2y + 4z$$

D

$$2y \sin x - z^2 \cos x - 2y + 4z$$