Structures of four disaccharides are given below. Among the given disaccharides, the non-reducing sugar is :

A disaccharide will behave as a reducing sugar only when **at least one** of its two monosaccharide units retains a free (hemi-acetal/ hemi-ketal) $$-OH$$ group at the anomeric carbon. Such a free anomeric $$-OH$$ can open up to the aldehydic or ketonic form and reduce mild oxidising agents like Tollens’ or Fehling’s reagents.

If **both** anomeric carbons become part of the glycosidic bond, the molecule now contains two acetal (or ketal) linkages and no free hemi-acetal/ hemi-ketal group. Then the sugar cannot get oxidised and is termed a non-reducing sugar.

Among the common disaccharides:

• maltose : $$\alpha$$-D-glucose ($$1 \rightarrow 4$$) $$\beta$$-D-glucose  → free anomeric $$C\!-\!1$$ on the second glucose ⇒ reducing
• lactose : $$\beta$$-D-galactose ($$1 \rightarrow 4$$) $$\beta$$-D-glucose  → free anomeric $$C\!-\!1$$ on glucose ⇒ reducing
• sucrose : $$\alpha$$-D-glucose ($$1 \rightarrow 2$$) $$\beta$$-D-fructose  → **both** anomeric carbons ($$C\!-\!1$$ of glucose and $$C\!-\!2$$ of fructose) are locked in the glycosidic bond ⇒ non-reducing
• cellobiose : $$\beta$$-D-glucose ($$1 \rightarrow 4$$) $$\beta$$-D-glucose  → free anomeric $$C\!-\!1$$ on the second glucose ⇒ reducing

Inspecting the given structures, only the structure in Option B shows a $$1 \rightarrow 2$$ linkage between the anomeric carbon of an $$\alpha$$-D-glucopyranosyl unit and the anomeric carbon of a $$\beta$$-D-fructofuranosyl unit—exactly the structural feature of sucrose. Since both anomeric centres participate in the bond, there is no free hemi-acetal/ hemi-ketal group.

Therefore, the sole non-reducing disaccharide in the list is sucrose.

Option B which is: sucrose