Which type of waste degradation occurs without oxygen?

Anaerobic decomposition is the correct answer because it specifically refers to the process by which organic matter breaks down in the absence of oxygen. This type of degradation is facilitated by microorganisms that thrive in oxygen-free environments, such as in landfills or deep soil layers. During anaerobic decomposition, these microorganisms decompose complex organic materials into simpler compounds, resulting in byproducts such as methane, carbon dioxide, and other organic acids. This process is an essential part of waste management as it influences how waste is stored and managed in landfills and can contribute to energy recovery through methane capture.

The other types of degradation and treatment listed do not occur in the absence of oxygen. Aerobic decomposition requires oxygen for microorganisms to break down organic material, chemical treatment involves the use of chemicals to stabilize or break down waste and generally doesn't focus on microbial processes, and physical processing refers to mechanical methods of handling waste like shredding or compacting, rather than biological decomposition processes.