The correct answer is B. lactic acid.
Lactic acid is a type of organic acid that is produced when bacteria break down lactose in milk. It is responsible for the sour taste of sour milk.
Glycolic acid is a type of alpha hydroxy acid that is used in skin care products. It is not found in sour milk.
Citric acid is a type of carboxylic acid that is found in citrus fruits. It is not found in sour milk.
Tartaric acid is a type of dicarboxylic acid that is found in grapes and wine. It is not found in sour milk.