The correct option is A Epistasis
Epistasis is a phenomenon that consists of the effect of one gene being dependent on the presence of one or more modifier genes. Similarly epistatic mutations have different effects in combination than individually. It was originally a concept from Genetics. It arises due to interactions, either between genes or within them leading to non-additive effects. In epistasis, two independent non-allelic genes affects the same trait of an individual in such a manner that one overmasks the expression of the other.