Stone fly nymphs live in beds of streams that contain a high level of dissolved oxygen (DO). When the dissolved oxygen decreases in the stream, stone fly nymphs can no longer live there. Stone fly nymphs are an example of

Indicator species

Explanation:

Indicator species refers to the species whose presence or absence in particular ecosystem indicates about the condition of the ecosystem.

In the given question, the stonefly which cannot survive if the dissolved oxygen in the water gets reduced as a result of the pollution indicates that the ecosystem or habitat has some pollutants which have decreased the level of DO in the aquatic habitat.

Thus, the indicator species is the correct answer.