a. nature

b. excessive use of resources

c. humans

d. natural disasters

Answer: c. humans

Explanation: The Non-degradable waste refers to toxic substances that are not damaged or naturally decomposed into less harmful compounds. Humans are a major source of sustainable waste. These pollutants include heavy metals such as arsenic, mercury and lead, polythene, synthetic fibers and DDT (dichloro-diphenyl-trichloroethane).