Which among the following is dicarboxylic acid?

To identify the dicarboxylic acid, we examine the carboxyl group count for each compound.

Valeric acid (pentanoic acid) has formula $\text{CH}_3(\text{CH}_2)_3\text{COOH}$, containing one carboxyl group and is thus monocarboxylic.

Caproic acid (hexanoic acid) is $\text{CH}_3(\text{CH}_2)_4\text{COOH}$, also monocarboxylic.

Glutaric acid (pentanedioic acid) has structure $\text{HOOC}(\text{CH}_2)_3\text{COOH}$, with two carboxyl groups, making it dicarboxylic.

Butyric acid (butanoic acid) is $\text{CH}_3(\text{CH}_2)_2\text{COOH}$, again monocarboxylic.

The dicarboxylic acid is glutaric acid.