Answer: C. Species
A species is a group of organisms in which members are capable of exchanging their genes or interbreeding. The organisms belonging to the same species exhibit many similar characteristics, so that they can be grouped into a species. They resemble each other, morphologically and anatomically, they interbreed to produce viable fertile offsprings. They share a common gene pool. They exhibit similar genetic material and karyotype. They also exhibit a common ancestor.
The species is the largest group of organisms that can interbreed.