Lactose on hydrolysis gives:

The correct answer is Option (4) → β-D-glucose and β-D-galactose

A disaccharide is made of two monosaccharide units joined by a glycosidic linkage, and hydrolysis breaks this linkage to give the two units back. Lactose, the milk sugar, is composed of \( \beta \)-D-galactose and \( \beta \)-D-glucose, linked between C1 of galactose and C4 of glucose:

    \( \displaystyle \ce{C12H22O11 + H2O} \xrightarrow{\ce{H+}\ \text{or lactase}} \beta\text{-D-galactose} + \beta\text{-D-glucose} \)

The C1 of the glucose unit is free, so lactose is a reducing sugar.

Why the other options are incorrect

• Option (1) \(\alpha \)-D-glucose and \(\alpha \)-D-galactose — Incorrect. Both anomers are wrong; in lactose both units are \( \beta \).

• Option (2) \(\alpha \)-D-glucose and \(\beta \)-D-galactose — Incorrect. The glucose unit in lactose is the \( \beta \) anomer, not \( \alpha \).

• Option (3) \(\beta \)-D-glucose and \(\alpha \)-D-galactose — Incorrect. The galactose unit in lactose is the \( \beta \) anomer, not \( \alpha \).

NCERT (2026-27 edition):
Lactose

Note: The same/similar question has been asked in different years by NTA as per the details given below: