Which of the following sugars will not reduce Fehling's reagent?

The correct answer is Option (2) → Sucrose

Sugars that can reduce Fehling's reagent are called reducing sugars. These sugars contain a free aldehyde group or a free hemiacetal group, which can open to form an aldehyde in solution.

Fehling's reagent is reduced by such sugars, producing a brick-red precipitate of Cu₂O.

Maltose -- Maltose has a free anomeric carbon capable of opening to form an aldehyde group. Therefore it is a reducing sugar and reduces Fehling's reagent.

Sucrose - Sucrose is formed by linkage between: Glucose (C1) — Fructose (C2)

Both anomeric carbons are involved in glycosidic bond formation, so no free aldehyde or keto group is available. Therefore sucrose is a non-reducing sugar and does not reduce Fehling's reagent.

Lactose Lactose contains a free anomeric carbon, allowing it to open into an aldehyde form. Thus it is a reducing sugar.

Fructose Fructose is a ketose, but in alkaline medium (Fehling's solution) it undergoes tautomerization to glucose and mannose, which contain aldehyde groups. Thus fructose reduces Fehling's reagent.