`Cu^(2+)(aq.)+X^(-)(aq.)overset(R.T.)toCuX darr+X_(2)`
'X' cannot be:

Options

A. `Cl^(-)(aq.)`
B. `I^(-)(aq.)`
C. `CN^(-)(aq.)`
D. `SCN^(-)(aq.)`

Explanation

To solve the question, we need to analyze the reaction provided and determine which of the given options cannot be represented by 'X' in the reaction:

**Reaction:**
\[ \text{Cu}^{2+}(aq) + \text{X}^-(aq) \overset{R.T.}{\rightarrow} \text{CuX} + \text{X}_2 \]

**Options:**
A) Cl⁻
B) I⁻
C) CN⁻
D) SCN⁻

### Step-by-step Solution:

1. **Understand the Role of Cu²⁺:**
- Cu²⁺ is a strong oxidizing agent. It can oxidize other species, meaning it can take electrons from them.

2. **Analyze Each Option:**
- **Option A: Cl⁻**
- Chloride ion (Cl⁻) is a weaker reductant. It does not readily oxidize to Cl₂ under normal conditions. Therefore, it cannot be oxidized by Cu²⁺ to form Cl₂.
- **Option B: I⁻**
- Iodide ion (I⁻) can be oxidized to iodine (I₂). Since I⁻ has a stable oxidation state of 0, it can participate in the reaction, making it a valid choice for X.
- **Option C: CN⁻**
- Cyanide ion (CN⁻) can be oxidized to cyanogen (C₂N₂ or CN₂). Thus, it can also be a valid choice for X.
- **Option D: SCN⁻**
- Thiocyanate ion (SCN⁻) can be oxidized to thiocyanogen (S₂CN₂). This means SCN⁻ can also be a valid choice for X.

3. **Conclusion:**
- Since Cl⁻ cannot be oxidized to Cl₂ and is a weaker reductant, it cannot be X in the reaction. Therefore, the answer is:
- **Answer: A) Cl⁻**