I think question is repeated in the same test...
lactic acid is present in milk and related products...hence correct answer is B

Explanation:
Sour milk is a type of milk that has undergone acidification due to the presence of bacteria. The bacteria convert lactose, the natural sugar in milk, into lactic acid through a process called fermentation. Lactic acid is responsible for the sour taste and smell of sour milk.

Details:
Sour milk is produced when certain bacteria, such as Lactobacillus, ferment lactose in milk, converting it into lactic acid. This acidification process results in the characteristic sour taste and smell of sour milk.

Importance of Lactic Acid:
Lactic acid is an organic compound with the chemical formula C3H6O3. It is a weak acid and is commonly found in fermented foods and beverages. Lactic acid has several important functions and properties:

1. Sour Taste: Lactic acid contributes to the sour taste of sour milk. The acid activates sour taste receptors on the tongue, resulting in the perception of sourness.

2. Preservation: The presence of lactic acid in sour milk helps to preserve it by creating an acidic environment that inhibits the growth of harmful bacteria. The acid lowers the pH of the milk, making it less conducive to bacterial growth.

3. Digestion: Lactic acid aids in digestion by breaking down proteins and promoting the absorption of nutrients. It also helps to regulate the pH balance in the digestive system.

4. Health Benefits: Lactic acid has been associated with several health benefits, such as improving gut health, boosting the immune system, and promoting the growth of beneficial gut bacteria.

Conclusion:
In conclusion, lactic acid is the main acid present in sour milk. It is produced through the fermentation of lactose by bacteria, resulting in the sour taste and smell. Lactic acid plays a crucial role in preserving the milk, aiding digestion, and offering potential health benefits.