Which is a redox reaction :

Options

A. `2CuI_(2)rarrCuI+I_(2)`
B. `NaCl+AgNO_(3)rarrAgCl+NaNO_(3)`
C. `NH_(4)Cl+NaOHrarrNH_(3)+NaCl+H_(2)O`
D. `Cr_(2)(SO_(4))_(3)+6KOHrarr2Cr(OH)_(3)+3K_(2)SO_(4)`

Answer

To determine which reaction is a redox reaction, we need to analyze the oxidation states of the elements involved in each reaction. A redox reaction involves both oxidation (increase in oxidation state) and reduction (decrease in oxidation state).

### Step-by-Step Solution:

1. **Identify the Reactions**: We need to look at the given reactions and identify the oxidation states of the elements involved.

2. **Analyze the First Reaction**:
- Reaction: Cu²⁺ + I⁻ → Cu⁺ + I₂
- Oxidation states:
- Copper (Cu) changes from +2 to +1 (reduction).
- Iodine (I) changes from -1 to 0 (oxidation).
- Since both oxidation and reduction occur, this is a redox reaction.

3. **Analyze the Second Reaction**:
- Reaction: NaCl + AgNO₃ → AgCl + NaNO₃
- This is a double displacement reaction.
- Oxidation states:
- Na: +1, Cl: -1, Ag: +1, N: +5, O: -2 remain unchanged.
- No change in oxidation states indicates that this is not a redox reaction.

4. **Analyze the Third Reaction**:
- Reaction: NH₄Cl + NaOH → NaCl + NH₃ + H₂O
- Oxidation states:
- Na: +1, Cl: -1, N: -3, H: +1 remain unchanged.
- No change in oxidation states indicates that this is not a redox reaction.

5. **Analyze the Fourth Reaction**:
- Reaction: K₂Cr₂O₇ + H₂SO₄ + Zn → Cr₂(SO₄)₃ + ZnSO₄ + H₂O
- Oxidation states:
- Chromium (Cr) changes from +6 to +3 (reduction).
- Zinc (Zn) changes from 0 to +2 (oxidation).
- Since both oxidation and reduction occur, this is also a redox reaction.

6. **Conclusion**:
- The first reaction (Cu²⁺ + I⁻ → Cu⁺ + I₂) and the fourth reaction (K₂Cr₂O₇ + H₂SO₄ + Zn → Cr₂(SO₄)₃ + ZnSO₄ + H₂O) are redox reactions.

### Final Answer:
The redox reactions among the given options are the first and fourth reactions.