In lactose, the reducing part is

Lactose is a disaccharide of glucose (reducing part) and galactose (non-reducing part). The reducing part is the part of the compound that has the anomeric carbon free which is C-1 in the case of glucose, which is free from linkage in the disaccharide lactose.

The structure of lactose is as follows.

Hence, the correct answer is option B) Glucose.