Phosphorus forms a number of oxoacids which differ in their structures and oxidation state of phosphorus. All the acids contain phosphorus atom//atoms linked tetrahedrally to four other atoms or groups. Each of them has at least one `P=O` or `P rarr O` unit and one `P-OH` unit. The `OH` group is ionisable but `H` atom linked directly to `P` is non-ionisabl. Structures of all the acids are considered to be derived either from phosphorus acid or phosphoric acid.
Which one is monobasic acid ?

Options

A. `H_3 PO_2`
B. `H_3 PO_3`
C. `H_3 PO_4`
D. `H_3 PO_5`

Explanation

To determine which of the given oxoacids of phosphorus is a monobasic acid, we need to analyze the structures and basicity of the acids mentioned. A monobasic acid has one ionizable hydroxyl (OH) group.

### Step-by-Step Solution:

1. **Identify the Oxoacids of Phosphorus**:
The oxoacids of phosphorus mentioned are:
- H₃PO₂ (Phosphorous acid)
- H₃PO₃ (Phosphoric acid)
- H₃PO₄ (Phosphoric acid)
- H₃PO₅ (Perphosphoric acid)

2. **Analyze H₃PO₂**:
- Structure: H₃PO₂ has one P=O bond and one P-OH bond.
- Basicity: It has one ionizable OH group and one non-ionizable hydrogen atom directly linked to phosphorus.
- Conclusion: H₃PO₂ is a **monobasic acid**.

3. **Analyze H₃PO₃**:
- Structure: H₃PO₃ has one P=O bond and two P-OH bonds.
- Basicity: It has two ionizable OH groups.
- Conclusion: H₃PO₃ is a **dibasic acid**.

4. **Analyze H₃PO₄**:
- Structure: H₃PO₄ has one P=O bond and three P-OH bonds.
- Basicity: It has three ionizable OH groups.
- Conclusion: H₃PO₄ is a **tribasic acid**.

5. **Analyze H₃PO₅**:
- Structure: H₃PO₅ has one P=O bond and three P-OH bonds, along with a peroxide linkage.
- Basicity: It has three ionizable OH groups.
- Conclusion: H₃PO₅ is a **tribasic acid**.

### Final Conclusion:
Among the acids analyzed, **H₃PO₂** is the only monobasic acid.