Breed
A breed refers to a group of animals that share common features such as shape, size, appearance and are related by descent.
Race
A race is a group of human beings that are categorized on the basis of various heritable characteristics like color of skin, hair, and eye.
Variety
Variety is a taxonomic group present below that of species and sub-species. A variety is a group of individuals with different genetic characteristics that inter breed with other groups of the same species.
Species:
A species is a group of individuals with the same characteristics that can mate with each other and produce fertile offspring.
Final answer: a. breed.