To ferment refers to which of the following processes?

Fermentation is a biochemical process in which microorganisms such as yeast or bacteria convert sugars and other carbohydrates into alcohol or acids under anaerobic conditions, meaning without oxygen. This process is essential in various industries, particularly in food and beverage production, where it is used to create products like beer, wine, yogurt, and bread.

The correct answer directly highlights the concept of fermenting as undergoing fermentation, which encapsulates the fundamental definition of the term. Understanding this process is crucial for recognizing its applications in both culinary contexts and biological systems, where it plays a critical role in metabolism and energy production for various organisms.

The other choices do not accurately reflect the scientific definition of fermentation. Calming and soothing, becoming stale, and freezing are unrelated processes and do not involve the conversion of substances resulting in the characteristic chemical changes associated with fermentation. This distinction emphasizes the unique and specific nature of fermentation in both natural and industrial contexts.