In the new six kingdom system of classification, the six kingdoms may be grouped into three domains. the domain archaea contains organisms that are a) eukaryotic consumers. b) composed of one or more eukaryotic cells. c) prokaryotic cells living in extreme environments. d) prokaryotic cells that are typical of disease-causing bacteria.

The right option is; C) prokaryotic cells living in extreme environments.

The domain archaea contains organisms that are prokaryotic cells living in extreme environments.  

The domain archaea contains unicellular prokaryotic organisms (organisms whose cells do not have a true nucleus). These organisms have definite attributes that separates them from bacteria and eukaryotes and they mostly live in extreme environments such as hydrothermal vents, volcanic hot springs, and in salty environments like the Dead Sea. Examples of archaea include methanogens, halobacteria and sulfolobus.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."