The biological species concept is the most widely accepted species concept. Ernst Mayr defined a species as 'species are groups of interbreeding natural populations that are reproductively isolated from other such groups'.
So, the correct option is 'Species'.

Explanation:

Species is the basic unit of classification and interbreeding is one of the most important criteria for defining a species. Therefore, individuals of one population can interbreed with individuals of another population only if they belong to the same species.

- Definition of Species:
A species is a group of organisms that can interbreed to produce fertile offspring in nature and are reproductively isolated from other such groups. The concept of species is based on the biological species concept, which defines a species as a group of interbreeding individuals that are reproductively isolated from other such groups.
- Reproductive Isolation:
Reproductive isolation refers to the inability of individuals from different populations to interbreed or produce fertile offspring. This can occur due to pre-zygotic or post-zygotic barriers. Pre-zygotic barriers prevent mating or fertilization from occurring, while post-zygotic barriers prevent the development or survival of hybrid offspring.
- Importance of Interbreeding:
Interbreeding is important because it maintains genetic continuity within a species and prevents the formation of new species. Without interbreeding, different populations can evolve independently and become genetically distinct, which can ultimately lead to the formation of new species.
- Examples:
For example, dogs and wolves are different species because they cannot interbreed to produce fertile offspring in nature. However, different breeds of dogs are still considered the same species because they can interbreed and produce fertile offspring. Similarly, horses and donkeys are different species, but they can interbreed to produce infertile mules.