Which of these compounds is classified as a disaccharide?

The classification of carbohydrates is based on the number of sugar units they contain. A disaccharide is specifically a carbohydrate composed of two monosaccharide units linked together by a glycosidic bond.

Lactose, which is identified as a disaccharide, consists of one glucose and one galactose molecule. Sucrose, also a disaccharide, is made up of one glucose unit and one fructose unit, making it a prime example of this classification.

On the other hand, glucose and fructose are both classified as monosaccharides, which are the simplest form of carbohydrates and consist of single sugar molecules. Hence, while lactose and sucrose both qualify as disaccharides, the specific request in the question for a disaccharide identifies sucrose as the suitable answer due to its broad recognition and use in various contexts.