The Acid Present in Curd/Sour Milk is (1) Acetic acid (2) Formic acid (3) Lactic acid (4) Hydrochloric acid.

Answer: The correct answer here is an option (3) Lactic Acid.

Yogurt or curd is mainly formed due to the milk protein casein and lactic acid bacteria reaction. There is lactic acid present in the curd or sour milk. The sourness that we get in the milk is because of the lactic acid. 

Milk acid or lactic acid is mainly an organic acid and the chemical formula is C3H603. When the lactose and milk sugar is fermented, it is then obtained the lactic acid. When the fermentation happens, the bacteria further produce ATP from the lactose. The lactic acid is the by-product of this ATP production during the curdling. It can be found in sour milk, koumiss, Leban, curd and cottage cheese.