The correct option is A Both assertion and reason are true and reason is the correct explanation of assertion
Lactic acid is responsible for the fresh acidic flavor of unripened cheese and is important in coagulation of milk casein, which is accomplished by the combined action of rennet (an enzyme) and lactic acid produced by the microbes.
The lactic series of cheese are primarily made with little to no rennet and rely primarily on the action of the bacteria converting the milk lactose to lactic acid. When the milk acidity becomes high enough, the milk will coagulate even without the use of rennet.