Final answer:
The process of decomposing organic waste with microorganisms such as bacteria and fungi is called composting. As decomposers, these organisms break down organic material, returning nutrients to the ecosystem. Hence, the correct answer to the student's question is 'Composting' (option A).
Step-by-step explanation:
The natural process of decomposing organic waste with the help of different microorganisms such as bacteria and fungi is known as composting. This biological process involves the breakdown of organic material by decomposers, which are typically bacteria and fungi. These organisms transform waste and dead materials into simple inorganic molecules that can then be recycled back into the ecosystem as nutrients for plants to use. Composting does not use burning (incineration) or rely on simply burying the waste (landfilling). Instead, it is a sustainable method to manage organic waste and is part of the important biogeochemical cycles that maintain ecosystem health.
Bioremediation is another related approach where organisms help treat contaminated environments, further emphasizing how vital these biological processes are to Earth's balance. The function of decomposers in recycling nutrients is essential for living organisms, and without them, ecosystems would face nutrient depletion. So, considering choices A to D from the student's question, Composting (option A) is the most appropriate answer.