The correct option is D Free energy
In an ecosystem biogeochemical cycles like cycle of carbon, potassium, nitrogen etc. are always cyclical. These nutrients are released back into the abiotic parts of the ecosystem with the death and decay of the biotic components. It is then reutilized by the producers and cycled back into the system. The flow of energy, however is always undirectional. Energy is lost at each trophic level in the form of heat and cannot be cycled back into the system.