Step 1: Meaning
A dicarboxylic acid contains two $-COOH$ groups.
Step 2: Analysis
- Valeric acid (5C), Caproic acid (6C), and Butyric acid (4C) are monocarboxylic acids.
- Glutaric acid is a 5-carbon dicarboxylic acid ($HOOC-(CH_2)_3-COOH$).
Step 3: Conclusion
Glutaric acid is the dicarboxylic acid.
Final Answer: (C)