Q.An assemblage of individuals of different species living in the same habitat and having functional interactions is ................

Step 1. A population consists of individuals of the SAME species, which rules out option (c). Step 2. A niche (b) is the functional role of a single species, not an assemblage of individuals, so that is ruled out too. Step 3. An ecosystem (d) additionally includes the abiotic environment interacting with the community, which is broader than what the question describes. Step 4. An assemblage of individuals of DIFFERENT species living together in one habitat and interacting functionally is, by definition, a biotic community.

Eliminate options against the precise definitions in the ecological hierarchy (organism -> population -> community -> biome).

• Mixing up "community" with "ecosystem" (an ecosystem = community + abiotic environment).
• Mistaking "population" (single species) for a multi-species assemblage.