Microorganisms involved in curd formation is

The correct option is B $Lactobacillus$
Lactobacillus belongs to the group of bacteria which perform fermentation and convert the sugars into lactic acid in the absence of oxygen. This bacterium can also be added to the milk in the form of already synthesized curd. Lactose is the disaccharide sugar present in the milk that gets converted to lactic acid. The lactic acid which is produced gives a sour taste and the milk is converted to curd.

Thus, the correct answer is option B.