`Cu^(2+)(aq.)` does not undergo redox reaction with solution:

Options

A. `(NH_(4))_(2)S`
B. `Na_(2)S_(2)O_(3)`
C. `KI`
D. `NH_(4)SCN`

Explanation

To determine which solution does not undergo a redox reaction with Cu²⁺ (aq), we will analyze each of the four solutions provided:

1. **Ammonium Sulfide (NH₄₂S)**:
- When Cu²⁺ reacts with NH₄₂S, it forms CuS.
- In this reaction, the oxidation state of copper remains +2 (Cu²⁺) and does not change.
- Since there is no change in the oxidation state, this is **not a redox reaction**.

2. **Sodium Thiosulfate (Na₂S₂O₃)**:
- When Cu²⁺ reacts with Na₂S₂O₃, it forms Cu₂S₂O₃.
- In this case, Cu²⁺ is reduced to Cu⁺ (oxidation state changes from +2 to +1).
- Since there is a change in the oxidation state, this is a **redox reaction**.

3. **Potassium Iodide (KI)**:
- When Cu²⁺ reacts with KI, it forms Cu₂I₂.
- Here, Cu²⁺ is reduced to Cu⁺ (oxidation state changes from +2 to +1).
- This indicates a change in oxidation state, so it is a **redox reaction**.

4. **Ammonium Thiocyanate (NH₄SCN)**:
- When Cu²⁺ reacts with NH₄SCN, it forms Cu(SCN).
- In this reaction, Cu²⁺ is reduced to Cu⁺ (oxidation state changes from +2 to +1).
- This is also a **redox reaction**.

**Conclusion**: The only solution that does not undergo a redox reaction with Cu²⁺ is **Ammonium Sulfide (NH₄₂S)**.

### Summary of Steps:
1. Analyze the reaction of Cu²⁺ with each solution.
2. Determine the oxidation states of copper before and after the reaction.
3. Identify whether there is a change in oxidation state (indicating a redox reaction).
4. Conclude which solution does not cause a redox reaction.