By which of the following milk becomes sour?

Milk becomes sour primarily due to the action of bacteria. Specifically, lactic acid bacteria such as Lactobacillus and Streptococcus ferment the lactose (milk sugar) present in the milk, converting it into lactic acid. This acidification changes the pH of the milk, leading to the sour taste and causing the proteins in the milk to coagulate, forming curds. These bacteria are naturally present in milk and other dairy products, and their activity is essential in the production of sour milk, yogurt, and cheese. The fermentation process not only changes the flavor and texture of the milk but also increases its shelf life by inhibiting the growth of spoilage organisms. While bacteria are the primary agents of milk souring, the other options, such as protozoa, viruses, and nematodes, do not play a significant role in this process.