Lactose is composed of:

The correct answer is Option (1) → β-D galactose and β-D glucose

A disaccharide is made of two monosaccharide units joined by a glycosidic linkage. The three common disaccharides are:

• Sucrose (cane sugar): α-D-glucose and β-D-fructose, joined between C1 of glucose and C2 of fructose; non-reducing sugar.

• Maltose (malt sugar): two α-D-glucose units, joined between C1 of one and C4 of the other; reducing sugar.

• Lactose (milk sugar): β-D-galactose and β-D-glucose, joined between C1 of galactose and C4 of glucose; reducing sugar.

So lactose is composed of β-D-galactose and β-D-glucose.

Why the other options are incorrect

• Option (2) α-D glucose and β-D galactose — Incorrect. The two sugars are right, but the glucose unit in lactose is β-D-glucose, not α-D-glucose.

• Option (3) α-D glucose and β-D fructose — Incorrect. These are the units of sucrose; lactose contains no fructose.

• Option (4) β-D glucose and α-D galactose — Incorrect. The galactose unit in lactose is β-D-galactose, not α-D-galactose.

NCERT (2026-27 edition):
Lactose

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