In blue solution of copper sulphate excess of `KCN` is added then solution becomes colourless due to the formation of :

Options

A. `[Cu(CN)_(4)]^(2-)`
B. `Cu^(2+)` get reduced to form `[Cu(CN)_(4)]^(3-)`
C. `Cu(CN)_(2)`
D. `CuCN`

Explanation

To solve the question regarding the addition of excess KCN to a blue solution of copper sulfate (CuSO4), we can break it down into the following steps:

### Step-by-Step Solution:

1. **Identify the Initial Solution**:
- The initial solution is a blue solution of copper sulfate (CuSO4). The blue color is due to the presence of Cu²⁺ ions in the solution.

2. **Addition of KCN**:
- When excess potassium cyanide (KCN) is added to the blue solution, it reacts with the copper ions (Cu²⁺) present in the solution.

3. **Formation of Copper(I) Cyanide**:
- The reaction between Cu²⁺ and KCN leads to the formation of copper(I) cyanide (CuCN). The reaction can be represented as:
\[ \text{Cu}^{2+} + 2 \text{CN}^- \rightarrow \text{CuCN} \, (\text{yellow precipitate}) \]
- Initially, a yellow precipitate of CuCN may form, but this compound is unstable.

4. **Decomposition of Copper(I) Cyanide**:
- The unstable CuCN can decompose to release cyanogen gas (C2N2) and further react with excess KCN:
\[ 2 \text{CuCN} \rightarrow 2 \text{Cu} + \text{C}_2\text{N}_2 \, (\text{cyanogen gas}) \]

5. **Formation of Tetracyanocuprate(I) Complex**:
- The remaining CuCN reacts with excess KCN to form a soluble complex known as tetracyanocuprate(I) ion, represented as:
\[ \text{CuCN} + 4 \text{CN}^- \rightarrow \text{Cu(CN)}_4^{3-} \]
- This complex is colorless, which explains the change in color of the solution from blue to colorless.

6. **Conclusion**:
- The color change from blue to colorless is due to the formation of the tetracyanocuprate(I) complex, which is soluble and does not impart color to the solution.

### Final Answer:
The solution becomes colorless due to the formation of the complex ion \(\text{Cu(CN)}_4^{3-}\).