During the production of yogurt, specific are used to ferment the milk. in turn, the same are beneficial to our digestive system when we eat the yogurt.
a) algae
b) bacteria
c) fungi
d) protozoan

Bacteria

Explanation:

During the process of curd formation specific kind of bacteria known as lactobacillus bacteria is used to ferment milk.

The fermented milk is known as curd. When milk is heated upto a temperature of  30-40 degrees centigrade and some curd is added to it, the lactobacillus in the curd starts multiplying and converts the lactose present in milk to lactic acid.

This lactobacillus bacteria is very good for the health of people as it reaches the stomach and helps in better digestion of food.