Horses, zebras, and donkeys are all different species. they are believed to have come from one common ancestor and just become more and more different due to their different environmental conditions. these animals may have evolved by

Equus is a genus of mammals in the family Equidae, which includes horses, donkeys, and zebras. Within Equidae, Equus is the only recognized extant genus, comprising seven living species. The term equine refers to any member of this genus, including horses.

And a little info why Equus is a genus of mammals in the family...If you wanted it.

multiple allele inheritance is traits controlled by more than 2 alleles (ex: pigeons having 3 alleles of one gene govern their feather color).   so the answer is a.