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.
The acid which forms two series of salts is.

Options

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

Explanation

To solve the question regarding the oxoacids of phosphorus that form two series of salts, we will analyze the given acids based on their structure and basicity.

### Step-by-Step Solution:

1. **Understanding the Question**:
The question asks for an oxoacid of phosphorus that can form two series of salts. To form two series of salts, the acid must be dibasic, meaning it can donate two protons (H⁺ ions).

2. **Identifying the Relevant Acids**:
The common oxoacids of phosphorus include:
- Phosphoric acid (H₃PO₄)
- Phosphorous acid (H₃PO₃)
- Hypophosphorous acid (H₃PO₂)
- Phosphine oxide (HPO₃)

3. **Analyzing Each Acid**:
- **H₃PO₄ (Phosphoric Acid)**:
- Structure: Contains one P=O and three P-OH groups.
- Basicity: It is tribasic (can donate three protons). Therefore, it cannot form two series of salts.

- **H₃PO₃ (Phosphorous Acid)**:
- Structure: Contains one P=O and two P-OH groups.
- Basicity: It is dibasic (can donate two protons). Hence, it can form two series of salts.

- **H₃PO₂ (Hypophosphorous Acid)**:
- Structure: Contains one P=O and one P-OH group.
- Basicity: It is monobasic (can donate one proton). Therefore, it cannot form two series of salts.

- **HPO₃ (Phosphine Oxide)**:
- Structure: Contains one P=O and one P-OH group.
- Basicity: It is also monobasic (can donate one proton). Thus, it cannot form two series of salts.

4. **Conclusion**:
After analyzing the basicity of the acids, we find that **H₃PO₃ (Phosphorous Acid)** is the only acid that is dibasic and can form two series of salts.

### Final Answer:
The acid which forms two series of salts is **H₃PO₃ (Phosphorous Acid)**.