Anaerobic digestion refers to:

• A. Decomposition of waste under anaerobic conditions to produce biogas and a compost-like residue
• B. Aerobic decomposition producing CO2
• C. Chemical precipitation
• D. Digestion that requires oxygen

Answer: (A)

Decomposing organic waste without oxygen, using microorganisms that thrive in an oxygen-free environment, is the essence of anaerobic digestion. In this process, the waste is broken down step by step inside a sealed, oxygen-free system, producing biogas (primarily methane and carbon dioxide) and a digested residue that can resemble compost.

That’s why the described idea fits best: decomposition under anaerobic conditions that yields biogas and a compost-like residue. The other descriptions don’t match because aerobic decomposition needs oxygen and mainly makes CO2 and water, chemical precipitation is a different treatment method, and digestion that requires oxygen contradicts the meaning of “anaerobic.”