Answer
$${\text{Any constant C}}$$
Work Step by Step
$$\eqalign{
& {\text{The derivative of any constant }}C{\text{ is }}\frac{d}{{dx}}\left[ C \right] = 0,{\text{ Then }} \cr
& {\text{the sets of antiderivatives of }}f\left( x \right){\text{ is any constant }}C \cr} $$
