To remove milk stone, . . . . . . . . used.

Answer: Option C

To remove milk stone Acids are used.
Milk stones are hard body that forms in a mammalian udder. A layer of scale that forms on milk-processing equipment, mainly formed of cations of calcium and magnesium.
Milkstone acts as an insulator in milk-heating equipment, and if not removed regularly it will reduce the rate of heat transfer and consequently the heating capacity of the equipment.
Nitric acid is the most commonly used for stone removal and pH stabilization.