Which microorganism is used in curd?

The correct answer is Option (3) → Lactobacillus

Curd is made by adding a little old curd to warm milk. The old curd carries Lactobacillus, a lactic acid bacterium, which multiplies and turns the milk sugar (lactose) into lactic acid. The acid makes the milk proteins clot, so the milk sets into curd and turns sour. These bacteria also raise the curd’s vitamin B₁₂ content.