When Option (2) Moderate hot cup of milk is added to buttermilk, it turns into curd after a few hours.

•     Option (2) is correct.

Lactobacillus bacterium is responsible for curd production. In warm milk, this bacterium grows and reproduces to make curd.

•    Option (1) is incorrect.

Hot milk kills the bacteria in the milk at the onset of the fermentation process, resulting in a curd that is not properly fermented.

• Option  (3) is incorrect.

The temperature of cold milk is not favorable to the growth of Lactobacillus bacteria, hence it will not ferment into curd.