The acid present in curd is lactic acid. Lactic acid is commonly found in milk related products...so the correct answer is A.

**Answer:**

The acid present in soured curd is lactic acid. Lactic acid is a type of organic acid that is produced by the bacteria present in curd during the process of fermentation. Fermentation is a metabolic process in which microorganisms convert carbohydrates into acids, gases, or alcohol.

**Explanation:**

- **Fermentation of milk:** The fermentation of milk is carried out by bacteria called lactobacilli. These bacteria convert lactose, the sugar present in milk, into lactic acid through the process of fermentation. This lactic acid gives curd its characteristic sour taste and tangy flavor.

- **Lactic acid:** Lactic acid is a type of carboxylic acid that is produced by the breakdown of carbohydrates. It is a colorless, syrupy liquid with a sour taste and a mild odor. Lactic acid is also found in other fermented food products such as yogurt, sauerkraut, and pickles.

- **Role of bacteria:** The lactobacilli bacteria present in curd produce lactic acid by breaking down the lactose sugar into simpler molecules. These bacteria convert glucose and galactose, the components of lactose, into lactic acid through a series of biochemical reactions.

- **Health benefits:** Lactic acid fermentation not only gives curd its distinct taste but also provides several health benefits. Lactic acid helps in improving digestion by promoting the growth of good bacteria in the gut. It also helps in the absorption of minerals like calcium and iron. Additionally, lactic acid has antimicrobial properties that inhibit the growth of harmful bacteria.

In conclusion, the acid present in soured curd is lactic acid. It is produced by the bacteria present in curd during the fermentation of lactose, giving curd its characteristic sour taste and tangy flavor.