Why is maltose a reducing sugar?

Hint: Reducing sugars is the one that can reduce Fehling’s test and Tollens reagent. They should have at least one free aldehyde group or ketone group in the structure. Maltose has one free aldehyde group; the other is used up in the formation of glycosidic linkage between the two glucose units which make up maltose.

Complete answer:
Reducing sugars are the type of sugar that acts as the reducing agent and can effectively donate electrons to some other molecule by oxidizing it is called reducing sugar. These sugars tend to act as the reducing agent since they bear either free aldehyde group \[\left( { - CHO} \right)\] or ketone group \[\left( { - CO - } \right)\]. Examples are glucose, fructose, glyceraldehyde, etc. Reducing sugars can reduce cupric ions of benedict solution or Fehling solution to cuprous ions.
Maltose, also known as maltose or malt sugar, is a disaccharide formed from two units of glucose joined with a \[\alpha \left( {1 \to 4} \right)\;\] bond. It falls under oligosaccharides. Glucose is a hexose: a monosaccharide containing six carbon atoms. The two glucose units are in the pyranose form are joined by an O-glycosidic bond, with the first carbon \[({C_1})\] of the first glucose linked to the fourth carbon \[({C_4})\] of the second glucose, indicated as \[\left( {1 \to 4} \right)\]. In the structure of maltose, carbon number one of the first glucose ring has a free aldehyde group which can reduce Fehling’s solution and Tollens reagent. Because of this maltose is a reducing sugar.
Maltose can be broken down to glucose by the maltase enzyme, which catalyzes the hydrolysis of the glycosidic bond. It has a sweet taste.

Note:
Maltose is extensively used in alcohol production. Free maltose as well as maltose formed by the digestion of starch in the mouth, can cause dental caries. It lacks sweetness and is not used as a sweetener. Instead, it is used in the malting of barley for the manufacturing of beer.