The answer to this is letter D. Niche refers to the type of roles that the various organisms play in order to keep the ecosystem in good, working order. If in case you do not understand this well yet, think about what you do in school. Do you play in a school band? Are you a singer? Your role in school is your niche.
This is the same with the organisms that can be found in the ecosystem. They have their own niches too. For example, is a certain organism a decomposer? Is it an organism that produces the food that consumers need? These are roles that organisms will continue playing as long as the ecosystem is working well.