Which insect is used as 'Ecological indicator'?

Answer: Option C

Plecoptera insect is used as 'Ecological indicator'.
Most insects like beetles, ants, honeybees, and butterflies are used as biological indicators in this study because they are sensitive to even a slight change in the environment and are also used in monitoring various pollutants in environment.
All species of Plecoptera are intolerant of water pollution, and their presence in a stream or still water is usually an indicator of good or excellent water quality.
Due to their nymph's requirement for well-oxygenated water, the species is very sensitive to water pollution. This makes them important indicators for water quality.