Assertion: When `H_(2)S` is passed through a solution containing `[Cu(CN)_(4)]^(2-) and [Cd(CN)_(4)]^(2-)` ions, only cadmium precipitates as CdS.
Reason: The oxidation state and coordination number of cadmium in `[Cd(CN)_(4)]^(2-)` are II and 4 respectively.

Options

A. If assertion is true but the reason is false
B. If assertion is false but the reason is true
C. If both assertion and reason are true and reason is the correct explanation of assertion
D. If both assertion and reason are true but reason is not the correct explanation of assertion

Answer

To solve the question, we need to analyze the assertion and reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that when hydrogen sulfide (H₂S) is passed through a solution containing [Cu(CN)₄]²⁻ and [Cd(CN)₄]²⁻ ions, only cadmium precipitates as cadmium sulfide (CdS).

2. **Analyzing the Complexes**:
- The two complexes in question are [Cu(CN)₄]²⁻ and [Cd(CN)₄]²⁻. Both contain cyanide (CN⁻) as a ligand.
- Copper (Cu) and cadmium (Cd) both have an oxidation state of +2 in these complexes.

3. **Behavior of H₂S with Complexes**:
- When H₂S is added to the solution, it can react with metal ions to form metal sulfides. However, the stability of the complexes plays a crucial role in whether a precipitate will form.
- [Cu(CN)₄]²⁻ is a more stable complex compared to [Cd(CN)₄]²⁻. This means that the copper complex is less likely to release Cu²⁺ ions into the solution when H₂S is added.

4. **Formation of Precipitate**:
- Since [Cu(CN)₄]²⁻ does not precipitate with H₂S, only cadmium ions from [Cd(CN)₄]²⁻ will be released into the solution, leading to the formation of a yellow precipitate of CdS.

5. **Understanding the Reason**:
- The reason given states that the oxidation state and coordination number of cadmium in [Cd(CN)₄]²⁻ are II and 4, respectively.
- This is indeed correct: the oxidation state of cadmium is +2, and with four CN⁻ ligands, the coordination number is 4.

6. **Evaluating the Reason's Relevance**:
- While the reason is true, it does not explain why only cadmium precipitates as CdS. The reason does not address the comparative stability of the copper and cadmium complexes in the presence of H₂S.

7. **Conclusion**:
- Therefore, the assertion is true, but the reason provided is not the correct explanation for the assertion. The correct explanation involves the relative stability of the complexes.

### Final Answer:
- Both assertion and reason are true, but the reason is not the correct explanation of the assertion.