The carboxylic acid which has maximum solubility in water is:

Options

A. phthalic acid
B. succinic acid
C. malonic acid
D. salicylic acid

Explanation

To determine which carboxylic acid has the maximum solubility in water among the given options (thalic acid, succinic acid, melonic acid, and salicylic acid), we can analyze the structures and the presence of hydrophilic and hydrophobic parts in each molecule.

### Step-by-Step Solution:

1. **Identify the Structures of the Acids**:
- **Thalic Acid**: Contains a benzene ring with two carboxylic acid groups (C6H4(COOH)2).
- **Succinic Acid**: Has a straight-chain structure with two carboxylic acid groups (HOOC-CH2-CH2-COOH).
- **Melonic Acid**: Similar to succinic acid but with one additional methylene group (HOOC-CH2-COOH).
- **Salicylic Acid**: Contains a benzene ring with one carboxylic acid group and one hydroxyl group (C6H4(OH)(COOH)).

2. **Analyze Hydrophilic vs. Hydrophobic Parts**:
- **Thalic Acid**: The benzene ring is a bulky hydrophobic part, which reduces solubility.
- **Succinic Acid**: Contains a straight-chain structure, which is more hydrophilic than thalic acid but still has some hydrophobic character.
- **Melonic Acid**: Contains only one methylene group (CH2), making it the least hydrophobic among the options, thus more hydrophilic.
- **Salicylic Acid**: The presence of the benzene ring contributes to its hydrophobic character, despite having a hydroxyl group.

3. **Consider Hydrogen Bonding**:
- The solubility of carboxylic acids in water is largely influenced by their ability to form hydrogen bonds. More hydrogen bonding leads to higher solubility.
- Melonic acid, having the least hydrophobic character and the ability to form hydrogen bonds through its carboxylic groups, will exhibit the highest solubility in water.

4. **Conclusion**:
- Among the four acids, melonic acid has the least hydrophobic part and the greatest potential for hydrogen bonding due to its structure. Therefore, it will have the maximum solubility in water.

### Final Answer:
The carboxylic acid which has maximum solubility in water is **melonic acid** (Option C).