Answer:
The correct answer is - warning system.
Step-by-step explanation:
Warning systems are a type of system that could be technical or biological in nature that can help in assessing or inform about the possible danger or change in a particular setting such as ecosystem, weather, water quality, and many others. It helps in avoiding such threats before occurring or become serious threats.
Certain biological species that live or require a particular set of conditions for their survival and reproduction can act as a biological warning system for water pollution, air pollution.