Which of the following is NOT a definition of a pollutant?

A pollutant is typically defined as a substance that can cause harm to the environment, health, or aesthetic value. The correct answer indicates that a substance which enhances the quality of an ecosystem does not fit the definition of a pollutant.

In ecological terms, substances that enhance the quality of ecosystems would contribute positively, supporting biodiversity or improving environmental conditions. Conversely, the other definitions listed describe types of pollutants: those that can be harmful to health, those that diminish aesthetic quality, and those that are present in excessive amounts, all of which align with the common understanding of pollutants as detrimental to health or the environment.

Hence, a substance that enhances rather than diminishes environmental quality clearly does not conform to the typical definitions of a pollutant.