A true species comprises of a population

The correct option is Reproductively isolated.

According to the biological species concept, a true biological species is defined as a group of individuals or populations that are capable of interbreeding with one another in nature to produce fertile offspring, and are reproductively isolated from other such groups.
Reproductive isolation is the key mechanism that prevents members of different species from producing offspring, or ensures that any offspring produced are sterile. This boundary maintains the genetic integrity of the species over time.

Let is break down why reproductive isolation is the defining feature of a true species:
1. Preservation of the Gene Pool: By preventing interbreeding with other species (gene flow), reproductive isolation ensures that the unique genetic traits of a population are preserved and passed down to future generations without being diluted.
2. Independent Evolution: Because there is no genetic exchange with external populations, the reproductively isolated population accumulates its own mutations and undergoes natural selection independently, allowing it to diverge and form a distinct evolutionary lineage.
3. Isolation Barriers: These barriers can be pre-zygotic (preventing mating or fertilization, such as differences in mating calls, breeding seasons, or anatomical structures) or post-zygotic (preventing the hybrid offspring from developing or reproducing, such as hybrid sterility seen in mules).

Other options, such as sharing the same niche, interbreeding, or feeding on the same food, are characteristics that many species share, but they are not sufficient definitions. For instance, different species can share the same habitat or food sources, but they remain distinct species because they are reproductively isolated and cannot merge their gene pools.