Which type of selection on a polygenic trait increases variation in the trait

The answer is Mutations.  It is the type of selection on a polygenic trait that increases variation in the trait.  Mutations increase genetic variation, which is needed for natural selection to bring about evolutionary change.  When a cell divides, it makes a copy of its DNA — and sometimes the copy is not quite perfect. That small difference from the original DNA sequence is a mutation. 

Ithink its c but im not 100% sure