Which one of the following terms describes not only the physical space occupied by an organism, but also its functional role in the community of organisms?

The correct answer is B) Ecological niche. The question's key phrase is 'not only the physical space... but also its functional role', which is the standard definition of a niche. Habitat is the address; the niche is the address plus the role the organism plays there. Hutchinson's idea of the niche as a set of tolerances and resource requirements, and Odum's view of it as the organism's role in the community, both capture this.

Why the others fail:

• Ecotone is a transition zone between two communities, such as the edge where forest meets grassland. It describes a boundary, not an organism's role.
• Habitat is only the physical place where an organism lives. It fits the first half of the statement but misses the functional role, which is exactly what the question tests.
• Home range is the area an animal regularly moves through for feeding and breeding. It is a spatial idea with no functional meaning.

Mains connection: niche overlap and competitive exclusion help explain biodiversity loss and invasive species spread, which recur in GS-III environment answers.

Trap: candidates pick Habitat because it sounds closest. Habitat answers 'where'; niche answers 'where and what role'.