The correct option is A Vitamin B12
Curd is obtained by the fermentation of milk by lactic acid bacteria such as Lactobacillus.These microbes convert the lactose present in the milk, in the absence of oxygen, to form lactic acid.
The lactic acid lowers the pH of milk causing the coagulation and partial digestion of the milk proteins. Thus, curd is formed.
The lactic acid bacteria present in curd produces vitamin B12 which enhances its nutritional quality.