The functional group that can be identified through Tollens’ reagent test is:

The Tollens’ reagent consists of a solution of silver nitrate (AgNO₃) in ammonia, which forms the diamminesilver(I) complex [Ag(NH₃)₂]⁺. This reagent is a mild oxidizing agent.

When an aldehyde is added to Tollens’ reagent, the aldehydic carbonyl carbon is oxidized to a carboxylate ion while the silver(I) ion is reduced to metallic silver (Ag). The metallic silver deposits on the walls of the test tube, producing a characteristic “silver mirror” on the interior surface.

Reaction (simplified):

$RCHO\; +\; 2[Ag(NH$₃)₂]⁺ + 3OH^- → RCOO^- + 2Ag(s) + 4NH₃ + 2H₂O

Key points that allow us to identify the functional group:

1. **Aldehydes** possess the –CHO group, which is readily oxidizable under mild conditions. The Tollens’ test exploits this property, giving a positive result (silver mirror).

2. **Phenols** and **alcohols** contain –OH groups attached to aromatic rings or saturated carbons, respectively. These groups are not oxidized by Tollens’ reagent, so no silver mirror forms.

3. **Carboxylic acids** already represent the fully oxidized form of an aldehyde; they cannot be further oxidized by this mild reagent, and thus also give a negative result.

Therefore, the only functional group among the options that yields a positive Tollens’ test is the aldehyde group.

**Conclusion:** The functional group identifiable by the Tollens’ reagent test is the aldehyde.