Which of the following is a non-reducing sugar?

Sugars are classified as reducing or non-reducing depending on whether they can act as a reducing agent (i.e. whether they can reduce Tollens’ reagent, Fehling’s solution, etc.).
A sugar shows reducing behaviour when it possesses a free anomeric carbon bearing an $$-OH$$ group; in aqueous solution this carbonyl carbon can open out to the aldehyde (or α-hydroxy-ketone) form and supply the electrons needed for reduction.

Disaccharides formed by glycosidic linkage between the anomeric carbons of both monosaccharide units do not have any free anomeric carbon left. Consequently, their open-chain aldehyde/ketone forms cannot appear, so they fail to reduce the usual reagents and are called non-reducing sugars.

Let us examine each option:

Option A — Lactose
Lactose consists of $$\beta$$-D-galactopyranose linked to $$\beta$$-D-glucopyranose through $$\beta(1\rightarrow4)$$ glycosidic bond. Only the anomeric carbon of the galactose unit is involved; the anomeric carbon of the glucose unit remains free. Hence lactose can mutarotate and reduce Fehling’s/Tollens’ reagent. It is a reducing sugar.

Option B — Fructose
Fructose is a monosaccharide (a ketohexose). In basic medium it undergoes keto-enol tautomerism to give an aldehydic form capable of reduction. Therefore fructose is also a reducing sugar.

Option C — Sucrose
Sucrose is made of $$\alpha$$-D-glucopyranose and $$\beta$$-D-fructofuranose linked through $$\alpha(1\rightarrow2)\beta$$ glycosidic bond. Both anomeric carbons (C-1 of glucose and C-2 of fructose) participate in the linkage, so neither monosaccharide possesses a free anomeric carbon. Hence sucrose cannot open up to an aldehyde/ketone form and cannot reduce common chemical reagents. Sucrose is therefore a non-reducing sugar.

Option D — Maltose
Maltose is formed by $$\alpha$$-D-glucopyranose units via an $$\alpha(1\rightarrow4)$$ linkage. Only one anomeric carbon is involved; the other is free. Thus maltose can act as a reducing agent and is a reducing sugar.

Among the given choices, the only sugar lacking a free anomeric carbon is sucrose.

Hence the non-reducing sugar is:
Option C which is: Sucrose.