Which statement best describes anaerobic respiration?

• A. It occurs in the absence of oxygen and yields less energy than aerobic respiration.
• B. It uses oxygen to generate the most energy.
• C. It occurs only in plants.
• D. It produces more energy than fermentation.

Answer: (A)

The main idea here is energy yield without oxygen. Anaerobic respiration occurs when organisms harvest energy without using oxygen, by transferring electrons to a different, non-oxygen terminal acceptor. Because oxygen is the most efficient final electron acceptor, using other acceptors yields less energy than aerobic respiration. So describing it as happening in the absence of oxygen and producing less energy than aerobic respiration captures both the lack of oxygen and the reduced energy yield.

The other statements don’t fit as well: using oxygen and generating the most energy describes aerobic respiration; occurring only in plants is false since many bacteria and other organisms perform anaerobic respiration; and while anaerobic pathways often yield more energy than fermentation, that comparison is not the defining feature and isn’t universal, whereas the lack of oxygen and lower energy yield compared with aerobic respiration are.