Which is the reducing part in the lactose?

The correct answer is option 2. Glucose.

Lactose is a disaccharide composed of two monosaccharides: glucose and galactose. The reducing part of lactose is the glucose unit. Here's an explanation:

Structure of Lactose

Lactose is made up of one molecule of galactose and one molecule of glucose. The two monosaccharides are linked by a β-1,4-glycosidic bond.

Reducing Sugar

A reducing sugar is a sugar that can donate electrons to another molecule and thus reduce it. This property is generally due to the presence of a free aldehyde or ketone group. In the case of disaccharides, the reducing part is the monosaccharide that has a free anomeric carbon that can open up to form a free aldehyde group.

Lactose Breakdown

In lactose, galactose is attached to glucose via a β-1,4-glycosidic bond. The anomeric carbon of glucose (carbon 1) in the glycosidic bond is free and can open up to form a free aldehyde group when the glycosidic bond is broken.

Conclusion

In lactose, the glucose unit has a free anomeric carbon that can participate in reduction reactions. Hence, the reducing part of lactose is: Glucose.