Glucose reacts with Tollen's reagent (ammoniacal silver nitrate) due to the presence of

Aldehyde

The correct answer is Option (2) → Aldehyde
Glucose reacts with Tollen’s reagent due to the presence of an aldehyde (–CHO) group.
Tollen’s reagent is ammoniacal silver nitrate. It is a mild oxidising agent that specifically reacts with aldehydes. When an aldehyde is present, it gets oxidised to a carboxylic acid, and silver ions (Ag⁺) are reduced to metallic silver, forming a silver mirror on the test tube.

Glucose is generally shown in cyclic form, but in aqueous solution a small portion of glucose always converts into its open-chain form, which contains a free –CHO group.
This free aldehyde group is responsible for:

• Reducing Ag⁺ to Ag (silver mirror test)

• Making glucose a reducing sugar

Hence, the reaction occurs because of the aldehyde group in glucose.