Which one of the acids is a dibasic acid ?

Options

A. `H_3 PO_3`
B. `H_3 PO_2`
C. `HPO_3`
D. `H_3 PO_4`

Explanation

To determine which acid is a dibasic acid, we need to analyze the structures and basicities of the given acids. A dibasic acid is defined as an acid that can donate two protons (H⁺ ions) in an aqueous solution.

### Step-by-Step Solution:

1. **Identify the Acids**: The acids we need to consider are:
- H₃PO₃ (Phosphorous acid)
- H₃PO₂ (Hypophosphorous acid)
- H₃PO₄ (Phosphoric acid)
- HPO₃ (Metaphosphoric acid)

2. **Analyze H₃PO₃ (Phosphorous Acid)**:
- Structure: P(=O)(OH)₂H
- It has 2 hydroxyl (OH) groups.
- Basicity: The number of acidic hydrogens is 2 (from the two OH groups).
- Conclusion: H₃PO₃ is a dibasic acid.

3. **Analyze H₃PO₂ (Hypophosphorous Acid)**:
- Structure: P(=O)(OH)H₂
- It has 1 hydroxyl (OH) group.
- Basicity: The number of acidic hydrogens is 1.
- Conclusion: H₃PO₂ is a monobasic acid.

4. **Analyze HPO₃ (Metaphosphoric Acid)**:
- Structure: P(=O)(OH)H
- It has 1 hydroxyl (OH) group.
- Basicity: The number of acidic hydrogens is 1.
- Conclusion: HPO₃ is a monobasic acid.

5. **Analyze H₃PO₄ (Phosphoric Acid)**:
- Structure: P(=O)(OH)₃
- It has 3 hydroxyl (OH) groups.
- Basicity: The number of acidic hydrogens is 3.
- Conclusion: H₃PO₄ is a tribasic acid.

6. **Final Conclusion**:
- The only acid among those analyzed that has a basicity of 2 (and thus is dibasic) is H₃PO₃ (Phosphorous acid).

### Answer:
The dibasic acid is H₃PO₃ (Phosphorous acid).