Milk sugar, a disaccharide, is composed of

$β$-D-Galactose and $β$-D-Glucose

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

Milk sugar (lactose) is composed of β-D-Galactose and β-D-Glucose.

Lactose is a disaccharide found in milk. It is formed by:

• one unit of β-D-Galactose

• one unit of β-D-Glucose

They are linked by a β one to four glycosidic bond. THIS IS THE CORRECT ANSWER

Option 2: β-D-Galactose and α-D-Glucose

This option is close but not correct as written.
In lactose, the bond is formed from β-D-Galactose to glucose, but the glucose unit is not defined as α in the linkage description. The standard identification of lactose focuses on β linkage from galactose, not α-glucose. Hence this option is considered incorrect in such questions.

Option 3: α-D-Glucose and β-D-Fructose

This combination forms sucrose, not lactose.
Sucrose is common table sugar and is made of glucose and fructose.

Option 4: β-D-Glucose and α-D-Glucose

This represents two glucose units, which form disaccharides like maltose (α one to four linkage) or cellobiose (β one to four linkage depending on configuration).
It is not lactose because lactose must contain galactose.