`K_(4)[Fe(CN)_(6)]+M^(x+)(aq.)to underset("Coloured precipitate")(M_(4)[Fe(CN)_(6)]_(x)darr)`
Which of the following cation does not respond to the above reaction?

Options

A. `Cu^(2+)(aq.)`
B. `Fe^(3+)(aq.)`
C. `Zn^(2+)(aq.)`
D. None of these

Explanation

To solve the problem, we need to identify which cation does not respond to the reaction with the complex \( K_4[Fe(CN)_6] \) to form a colored precipitate. The relevant cations to consider are \( Cu^{2+}, Fe^{3+}, Zn^{2+} \), and we need to analyze their reactions with the given complex.

### Step-by-Step Solution:

1. **Understand the Reaction**: The reaction involves the complex \( K_4[Fe(CN)_6] \) reacting with a cation \( M^{x+} \) to form a new complex \( M_4[Fe(CN)_6] \) which is a colored precipitate.

2. **Identify the Cations**: The cations provided are \( Cu^{2+}, Fe^{3+}, Zn^{2+} \).

3. **Analyze Each Cation**:
- **Copper (\( Cu^{2+} \))**: When \( Cu^{2+} \) reacts with \( K_4[Fe(CN)_6] \), it forms a colored precipitate. Therefore, it responds to the reaction.
- **Iron (\( Fe^{3+} \))**: Similarly, \( Fe^{3+} \) also reacts with \( K_4[Fe(CN)_6] \) to form a colored precipitate. Thus, it also responds to the reaction.
- **Zinc (\( Zn^{2+} \))**: When \( Zn^{2+} \) reacts with \( K_4[Fe(CN)_6] \), it forms a precipitate that is described as bluish-white in color. This indicates that it does not form a colored precipitate but rather a colorless or very lightly colored precipitate.

4. **Conclusion**: Since \( Zn^{2+} \) does not form a colored precipitate, it is the cation that does not respond to the reaction with \( K_4[Fe(CN)_6] \).

### Final Answer:
The cation that does not respond to the above reaction is \( Zn^{2+} \).