Which group of micro-organisms grows in milk, converts it to curd and…

The correct answer is Option (2) → Lactic acid bacteria (LAB)

Micro-organisms such as Lactobacillus grow in milk and turn it into curd. They are called lactic acid bacteria (LAB), because the acids they produce coagulate and partly digest the milk proteins. They also raise the vitamin B₁₂ content of curd and, in our stomach, check disease-causing microbes.

Option (1) Methanogens — Incorrect. Methanogens are anaerobic bacteria that grow on cellulosic material and produce large amounts of methane, as in biogas plants and the rumen of cattle. They do not make curd.

Option (2) Lactic acid bacteria (LAB) — Correct. LAB such as Lactobacillus do all three things the question names: they grow in milk, turn it into curd and check disease-causing microbes in the stomach.

Option (3) Cyanobacteria — Incorrect. Cyanobacteria are autotrophic microbes, such as Anabaena, Nostoc and Oscillatoria, that fix atmospheric nitrogen. They act as biofertilisers in paddy fields.

Option (4) Nitrogen-fixing bacteria — Incorrect. Nitrogen-fixing bacteria, such as Rhizobium in the root nodules of legumes, add nitrogen to the soil as biofertilisers. They do not ferment milk.

NCERT Extract:
Microbes in Household Products – “… A common example is the production of curd from milk. Micro-organisms such as Lactobacillus and others commonly called lactic acid bacteria (LAB) grow in milk and convert it to curd. During growth, the LAB produce acids that coagulate and partially digest the milk proteins. A small amount of curd added to the fresh milk as inoculum or starter contain millions of LAB, which at suitable temperatures multiply, thus converting milk to curd, which also improves its nutritional quality by increasing vitamin B₁₂. In our stomach too, the LAB play very beneficial role in checking disease-causing microbes.”