**Species is a Closed Reproductive Group**

A species can be defined as a group of organisms that can interbreed to produce fertile offspring. This definition suggests that species form closed reproductive groups, indicating that members of the same species can only reproduce with other members of their own species. Therefore, the correct answer is A. Closed reproductive group.

**Definition of a Species**
A species is a fundamental unit of classification in biological taxonomy. It represents a group of organisms that share common characteristics and can interbreed to produce fertile offspring. This reproductive compatibility is a key criterion for defining a species.

**Reproductive Isolation**
Reproductive isolation is a mechanism that prevents different species from interbreeding. It can occur due to various factors such as geographical barriers, genetic incompatibility, or differences in mating behaviors. These isolating mechanisms maintain the integrity of species and prevent the exchange of genetic material between them.

**Closed Reproductive Group**
When members of a species can only reproduce with other members of their own species, it creates a closed reproductive group. This means that individuals from different species are unable to produce viable offspring. The reproductive barriers that prevent interbreeding can be prezygotic (occurring before fertilization) or postzygotic (occurring after fertilization).

**Prezygotic Barriers**
Prezygotic barriers include mechanisms that prevent mating or fertilization between individuals of different species. These barriers can be based on factors such as geographical isolation, differences in mating rituals, or incompatible reproductive structures.

**Postzygotic Barriers**
Postzygotic barriers occur after fertilization and prevent the production of viable offspring. These barriers can be due to genetic incompatibility, developmental abnormalities, or reduced hybrid fertility.

**Conclusion**
Based on the definition of a species and the concept of reproductive isolation, it is clear that a species is a closed reproductive group. Members of a species share reproductive compatibility, while individuals from different species are reproductively isolated. Therefore, the correct answer is A. Closed reproductive group.

Answer is (A)
because different species are unable to interbreed and produce healthy, fertile offspring due to barriers called mechanisms of reproductive isolation.