Which biogeochemical cycle includes the exoskeleton and shells of marine organisms as a reservoir? carbon nitrogen sulfur oxygen

The correct option is OXYGEN.
The biogeochemical cycle refers to the pathway by which a chemical element is moved through both the abiotic and biotic compartment of an ecosystem. Each element has its own cycle through which it is recycled.
Marine organisms in the biosphere usually create carbonated shells which are rich in oxygen, when these organisms die they serve as a source of molecular oxygen.