The particular part of an ecosystem where an organism/population lives is called its A. Ecosystem
B. Biome
D. Biosphere
C. Habitat

C. Habitat

Explanation:

Simply by knowing that D. Biosphere talks about what lives on land or what lives in water( like a whole Jungle or an whole ocean),  B. Biome ( I think ) means the whole Earth, and A. Ecosystem is out of the question because the question already tells you what an Ecosystem is...so C. Habitat. Also when thinking of a Habitat, think of how in a zoo, people try to create a small part of the area from their ecosystem.

for a different answer set the answer would be because the oxygen in the water had depleted (apex)