The correct option is D Lactobacillus
Milk is turned into curd by bacteria. Microorganisms such as Lactobacillus and others commonly called lactic acid bacteria (LAB) promote the formation of curd. Lactobacillus is a genus of bacteria which can convert lactose sugar into lactic acid by means of fermentation, eventually reducing the pH of the milk causing the coagulation of the casein protein thus leading to the formation of card.
Clostridium tetani is an obligate anaerobic gram negative bacterium which produces a toxin that causes a serious infection called tetanus. The toxins affect the brain and nervous system and that causes serious muscle spasm and many more difficulties.
Saccharomyces cerevisiae is a species of yeast that is principally used in alcoholic beverage and bread making industries as this yeast has the capability of fermenting sugar into alcohol and carbon dioxide.
Propionibacterium is a gram positive bacterium which has the capability of converting lactic acid into propionic acid with the help of trans-carboxylase enzyme.