Milk tastes sour when kept in the open for sometime due to the formation of A. Lactic acid B. Citric acid C. Acetic acid D. Carbonic acid

The correct answer is A. Lactic acid.

Milk tastes sour when kept in the open for sometime due to the formation of lactic acid. This is because the milk contains bacteria that convert lactose, a sugar found in milk, into lactic acid. The lactic acid gives milk its sour taste.

Citric acid is found in citrus fruits, such as lemons and limes. Acetic acid is found in vinegar. Carbonic acid is found in carbonated beverages.