Which of the following statement incorrect?

Options

A. `HNO_(2)` can act both as a reducing agent and as an oxidising agent but `HNO_(3)` acts only as an oxidising agent.
B. The oxidation number of phosphorus can vary from `-3" to "+5.`
C. The reaction between `NaOH` and `H_(2)SO_(4)` is a redox reaction.
D. Oxidation number can have positive, negative, zero values.

Answer

To solve the question of identifying the incorrect statement, we will analyze each option one by one.

### Step-by-Step Solution:

1. **Analyzing Option 1:**
- The statement claims that HNO2 can act both as a reducing agent and as an oxidizing agent, while HNO3 acts only as an oxidizing agent.
- **HNO3** has nitrogen in the +5 oxidation state, which means it can only act as an oxidizing agent because it cannot be oxidized further.
- **HNO2** has nitrogen in the +3 oxidation state, allowing it to either gain electrons (acting as an oxidizing agent) or lose electrons (acting as a reducing agent).
- **Conclusion:** This statement is correct.

2. **Analyzing Option 2:**
- The statement claims that the oxidation number of phosphorus can vary from -3 to +5.
- Phosphorus, being in Group 15, can lose 5 electrons to achieve a +5 oxidation state or gain 3 electrons to achieve a -3 oxidation state.
- Examples include PH3 (where phosphorus is -3) and H3PO4 (where phosphorus is +5).
- **Conclusion:** This statement is correct.

3. **Analyzing Option 3:**
- The statement claims that the reaction between NaOH and H2SO4 is a redox reaction.
- The reaction between NaOH (a base) and H2SO4 (an acid) is a neutralization reaction, producing Na2SO4 and water.
- There is no change in oxidation states of the elements involved, indicating that no oxidation or reduction occurs.
- **Conclusion:** This statement is incorrect.

4. **Analyzing Option 4:**
- The statement claims that the oxidation number can have positive, negative, and zero values.
- Oxidation numbers can indeed be positive (like +5 in HNO3), negative (like -3 in PH3), or zero (like in elemental forms).
- **Conclusion:** This statement is correct.

### Final Answer:
The incorrect statement among the options provided is **Option 3**.

---