The correct answer is option D. species.
It is the individuals of a species which can interbreed to produce fertile off springs. This concept is known as biological species concept. The individuals from different species cannot produce fertile off springs even if they mate. It happens because the eggs produced by the female of a particular species is compatible with the sperms of male of that particular species only. Even if through mating, sperm of a dog will reach the egg of a cat who belongs to another species, it will not be able to fuse with the egg. For fusion of egg and sperm their compatibility is must & they are compatible only if the male and female are of same species.
There are some exceptions to this concept like liger is an off spring which is produced when a male lion mates with a female tiger even though they both belong to different species. But, even though they are able to reproduce an off spring yet this off spring is not fertile.
