Answer : The correct answer for the blank is- C.reproductive isolation.
According to the biological species concept given by Ernst Mayor, species can be described as a group of organisms that are capable of interbreeding among themselves and produce viable and fertile offspring. Organisms from different species are thus reproductively isolated from members of other species. In other words, members from different species can not interbreed to produce viable and fertile offspring.
Thus, by morphology and reproductive isolation, organisms can be classified in the same species.
