Milk sugar, a disaccharide, is composed of

The correct answer is Option (1) → \(β\)-D-Galactose and \(β\)-D-Glucose

A disaccharide gives two monosaccharide units on hydrolysis; in the molecule the two units are joined by a glycosidic linkage. The three common disaccharides differ in their units and linkage:

• Sucrose (cane sugar): α-D-glucose and β-D-fructose, linked C1–C2; a non-reducing sugar.

• Maltose (malt sugar): two α-D-glucose units, linked C1–C4; a reducing sugar.

• Lactose (milk sugar).

Milk sugar is lactose, a reducing sugar made of one

Milk sugar is lactose; its units are given in the book passage.

beta;-D-galactose unit and one

Milk sugar is lactose; its units are given in the book passage.

beta;-D-glucose unit.

NCERT (2026-27 edition):
Disaccharides – “… (iii) Lactose: It is more commonly known as milk sugar since this disaccharide is found in milk. It is composed of β-D-galactose and β-D-glucose. The linkage is between C1 of galactose and C4 of glucose. Free aldehyde group may be produced at C-1 of glucose unit, hence it is also a reducing sugar.”