A(n) is a group of organisms that only reproduces with other members of their group.

The correct answer for the fill in the blank is Species.

A species can be defined as a group of organisms which are capable of reproducing with the members of their own group. They produce offsprings, which are viable and fertile. By doing this, these group of organisms become reproductively isolated ( as only interbreed with their own members)

This definition of species was given by Ernst Mayr under Biological species concept.

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