Apopulation of black cat has been interbreeding however in one litter a sole gray kitten is born the successive generations now have a few gray cat witch mutualism is responsible for introducing its new allele and the cat population

Explanation:A population of black cats has been interbreeding. However, in one litter, a sole gray kitten is born. The successive generations now have a few gray cats. Which mechanism is responsible for introducing this new allele in the cat population?

The Question, these are the answers I need help.

A.  

Gene selection

B.  

Gene mutation

C.  

Gene flow

D.  

Genetic drift

because  I do not know help.