What type of carbohydrate is lactose?

Lactose is classified as a disaccharide, which is a type of carbohydrate made up of two monosaccharide units. In the case of lactose, it consists of one glucose molecule and one galactose molecule linked together by a glycosidic bond. This distinguishing feature characterizes all disaccharides, which are formed from two monosaccharides.

The other options highlight different categories or substances that are not applicable to lactose. Polysaccharides, for instance, refer to large carbohydrate molecules composed of many monosaccharide units, such as starch or glycogen, which are structurally and functionally distinct from disaccharides. Enzymes are biological catalysts that facilitate chemical reactions, and while lactose is indeed a substrate for the enzyme lactase (which breaks it down), it does not classify lactose itself. The option that includes "all of the above" is incorrect because it assumes lactose fits into multiple categories that it does not. Therefore, identifying lactose specifically as a disaccharide provides the correct and precise classification.