Reducing sugars are carbohydrates that can act as reducing agents due to the presence of a free aldehyde or ketone group (or a free hemiacetal/hemiketal group).

• Galactose is a monosaccharide (an aldohexose) and possesses a free hemiacetal group, making it a reducing sugar.
• Gluconic acid is an oxidized form of glucose (a carboxylic acid) and is not a reducing sugar.
• $\beta$-methyl galactoside is a glycoside (an acetal) where the anomeric hydroxyl group is methylated. Since it lacks a free hemiacetal group, it cannot open to form an aldehyde and is therefore a non-reducing sugar.
• Sucrose is a disaccharide in which the anomeric carbons of both glucose and fructose are involved in the glycosidic bond, leaving no free reducing groups, making it a non-reducing sugar.