What does the term ‘inoculum’ mean when a small amount of curd is add…

The correct answer is Option (3) → A starter containing millions of LAB

Curd is made from milk by lactic acid bacteria (LAB) such as Lactobacillus. A small amount of curd is added to fresh milk to start the process. This small amount is called the inoculum or starter, and the useful part of it is the large number of LAB it carries, which multiply at a suitable temperature and turn the milk into curd.

Option (1) A starter rich in vitamin B₁₂ — Incorrect. Vitamin B₁₂ is not what the starter supplies. Curd becomes richer in vitamin B₁₂ as a result of LAB growth, but that is an effect of the process, not the meaning of inoculum.

Option (2) A starter rich in proteins — Incorrect. Milk itself provides the proteins. The acids made by LAB coagulate and partially digest these milk proteins; the starter is not added as a source of protein.

Option (3) A starter containing millions of LAB — Correct. The inoculum is a starter that contains millions of LAB. These bacteria multiply in the fresh milk and convert it into curd.

Option (4) A starter containing millions of yeast cells — Incorrect. Yeast (Saccharomyces cerevisiae) is used to ferment bread dough and beverages. Curd is formed by lactic acid bacteria, not by yeast.

NCERT Extract:
Microbes in Household Products – “… 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.”