The answer is c. One that is most easily affected by changes in water quality.
An indicator species is a biological species whose presence or absence and even abundance can indicate specific environmental conditions. For example, there are some species (plant, fish, invertebrates,...) that can live only in an extremely clean river. But, if that clean river gets polluted by human activity, the river will not be any long extremely clean, and sensitive species will not survive and will disappear from the river. Its absence will indicate that river quality is threatened.