A. Sucrose: Non-reducing sugar. The reducing groups (anomeric carbons) of both glucose and fructose are involved in the glycosidic linkage, leaving no free hemiacetal group to reduce Tollen's or Fehling's reagents.
B. Lactose: Reducing disaccharide. It retains a free anomeric carbon on its glucose unit.
C. Maltose: Reducing disaccharide. It retains a free anomeric carbon on one of its glucose units.
D. Galactose: Reducing monosaccharide. All monosaccharides (both aldoses and ketoses) possess free reducing groups and act as reducing agents.
Therefore, Lactose, Maltose, and Galactose are all reducing sugars.
