The only carboxylic acid which gives positive Tollen's test is

Formic acid

The correct answer is option 2. Formic acid.

The only carboxylic acid that gives a positive Tollen's test is Formic acid

Tollen's test is used to identify aldehydes. It involves the reduction of silver ions \((Ag^+)\) in Tollens' reagent by an aldehyde to form metallic silver \((Ag)\) as a precipitate.

Carboxylic acids typically do not have the functional group necessary for this reaction. They lack the readily oxidizable carbonyl carbon with a hydrogen atom \((C=O-H)\) present in aldehydes.

However, formic acid \((HCOOH)\) is an exception. Due to its structure, it exhibits a unique behavior:

Formic acid has both the carboxylic acid group \((COOH)\) and an aldehyde-like functionality \((CHO)\) within the same molecule.

In formic acid, the hydrogen atom directly attached to the carbonyl carbon \((CHO)\) can behave similarly to an aldehyde's hydrogen.

Therefore, formic acid can undergo oxidation in Tollen's reagent, losing its hydrogen and getting oxidized to carbon dioxide \((CO_2)\) while reducing the silver ions to metallic silver. This positive test allows us to distinguish formic acid from other carboxylic acids.

Other options:

Acetic acid \((CH_3COOH)\), oxalic acid \((HOOC-COOH)\), and phthalic acid \((C_6H_4(COOH)_2)\) are all carboxylic acids without an \(\alpha \)-hydrogen \((H)\) atom directly attached to the carbonyl carbon) and thus do not give a positive Tollen's test.