Which of the following is a reducing sugar?

We need to identify which of the given sugars is a reducing sugar.

A reducing sugar is a sugar that can act as a reducing agent because it has a free anomeric carbon (free aldehyde or ketone group, or a hemiacetal/hemiketal that can open to form one). Reducing sugars give a positive test with Tollens' reagent, Fehling's solution, and Benedict's solution.

Option A: Maltose

Maltose is a disaccharide made of two glucose units linked by an $$\alpha$$-1,4-glycosidic bond. One glucose unit has its anomeric carbon involved in the glycosidic bond, but the second glucose unit has a free anomeric carbon (C1 is in the hemiacetal form). This free anomeric carbon can open up to form a free aldehyde group, making maltose a reducing sugar.

Option B: Sucrose

Sucrose is a disaccharide of glucose and fructose linked by an $$\alpha,\beta$$-1,2-glycosidic bond. The glycosidic bond involves the anomeric carbons of both glucose (C1) and fructose (C2). Since neither sugar unit has a free anomeric carbon, sucrose is a non-reducing sugar.

Option C: Starch

Starch is a polysaccharide made of many glucose units. While starch technically has one reducing end, in practice, the ratio of reducing end to the total molecule is so small that starch does not give a positive reducing sugar test and is considered non-reducing.

Option D: Cellulose

Cellulose is also a polysaccharide of glucose units. Like starch, it has negligible reducing ability and is considered a non-reducing sugar.

Among the given options, only maltose has a free anomeric carbon that allows it to act as a reducing agent.

Hence, the correct answer is Option A: Maltose.