Which type of respiration occurs without oxygen?

Anaerobic respiration is the correct answer because it specifically refers to the process of generating energy in the absence of oxygen. This type of respiration allows organisms to convert nutrients into energy through pathways that do not require oxygen, often utilizing substrates such as glucose. Anaerobic respiration results in the production of compounds such as lactic acid or ethanol, depending on the organism and the biochemical pathways employed.

In contrast, aerobic respiration requires oxygen to effectively produce energy (ATP) and is the primary method of energy production in most higher organisms. ATP synthesis can occur through both anaerobic and aerobic pathways, but it is not a type of respiration in itself. Lastly, cellular respiration encompasses both aerobic and anaerobic processes, making it a broader category that includes anaerobic respiration as a subset. Thus, anaerobic respiration uniquely identifies the energy-generating process that occurs specifically without oxygen.