Asubstance that enters earth's cycles and does harm to living things is
a. freshwater.
b. hydrogen.
c. a pollutant.
d. oxygen.

If something is harmful then it means that it pollutes the cycles so the answer is C. a pollutant. It can either be in the form of carbon dioxide when we use cars so it pollutes air, or it can enter the hydrosphere as an oil spill and pollute the waters, or anything basically that brings harm to the cycles and thus also to us when we participate in these cycles.