Answer : (B) Sucrose
Step 1 — Recall what makes a sugar reducing
A sugar is reducing if it has a free aldehyde or ketone group (or a potential one, at a free anomeric carbon) that can reduce Tollens' reagent or Fehling's solution.
Step 2 — Examine sucrose
Sucrose is glucose joined to fructose through an $\alpha$-1,2-glycosidic linkage. This linkage involves :
- C-1 of glucose — its anomeric carbon
- C-2 of fructose — its anomeric carbon
Both anomeric carbons are locked into the linkage, so neither ring can open to expose a carbonyl group. Sucrose is therefore non-reducing.
Step 3 — Check the others
Glucose — a monosaccharide with a free $-CHO$ group ✓ reducing
Maltose — glucose + glucose through an $\alpha$-1,4 linkage; the second glucose keeps a free anomeric carbon ✓ reducing
Lactose — galactose + glucose through a $\beta$-1,4 linkage; again one anomeric carbon stays free ✓ reducing
Step 4 — Remember the general rule
$$\text{\textbf{All} monosaccharides are reducing sugars.}$$
$$\text{Among common disaccharides, only \textbf{sucrose} is non-reducing.}$$
On hydrolysis, however, sucrose gives glucose and fructose — both reducing. The product mixture is called invert sugar, because the sign of optical rotation reverses from $+66 \cdot 5^\circ$ to $-19 \cdot 9^\circ$.
