The vitamin whose content increases following the conversion of milk into curd by lactic acid bacteria is:

Lactic acid bacteria (LAB) convert milk into curd. These bacteria convert the lactose protein into lactic acid which causing the milk to coagulate and partial digestion of milk proteins to take place. This leads to increased vitamin B12 content.

The vitamins mentioned in the other options are not produced by lactic acid bacteria. Therefore, option c. vitamin B12 is the correct answer.