The fourth structure is a carboxylic acid
What is a Carboxyllic Acid ?
A carboxylic acid is an organic molecule in which a carbon (C) atom is double bonded to an oxygen (O) atom and a hydroxyl group (OH) is single bonded.
The carbon atom is linked to a hydrogen (H) atom or another univalent combining group by a fourth bond.
The general formula is --COOH
In the given structure , the structure which satisfies the above conditions , i.e. has a carbon atom linked to the two oxygen atom , one by a double bond and the other by a single bond , the oxygen atom that is not linked by a double bond is linked with a Hydrogen.
The fourth structure can be seen satisfying the above criteria and is a carboxylic acid.
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