Question

Monosaccharides are called reducing sugar but disaccharides are not. why

Answer 1

Reducing or non-reducing nature of carbohydrate is determined by presence or absence of free hemiacetal group respectively.

In monosaccharides, the hemiacetal group is free, and thus, all of them are reducing.
In disaccharides, hemiacetal group can participate in formation of glycoside bond. It is typical for sucrose, which is thus, a non-reducing sugar.
However, in some disaccharides, the hemiacetal group is not used for the formation of glycoside bond, and thus, they appear reducing. The examples of such sugars are lactose and maltose.