Which of the following processes is primarily anaerobic?

Fermentation is primarily an anaerobic process, meaning it occurs in the absence of oxygen. This process allows organisms, particularly certain bacteria and yeast, to convert sugars into energy when oxygen is not available. During fermentation, glucose is partially broken down to produce ATP, along with byproducts such as ethanol or lactic acid, depending on the organism and conditions.

In contrast, aerobic respiration, which requires oxygen, fully oxidizes glucose to produce a larger amount of ATP. Photosynthesis also involves aerobic processes, as it relies on oxygen produced during the light reactions. Cellular respiration typically refers to aerobic respiration in a broader context, which utilizes oxygen to maximize energy production. Thus, fermentation stands out as a distinct process primarily suited for anaerobic conditions, making it the correct answer.