Assertion (A): A is very dilute acidic solution of `Cd^(2+)` and `Ni^(2+)` gives yellow precipitate of `CdS` on passing hydrogen sulphide.
Reason (R) : Solubility product of `CdS` is more than that of `NiS`.

Options

A. If both (A) and (R) are correc, and (R) is the correct explanation of (A).
B. If both (A) and (R) are correct but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect, but (R) is correct.

Explanation

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

### Step 1: Analyze the Assertion
The assertion states that a very dilute acidic solution of \( \text{Cd}^{2+} \) and \( \text{Ni}^{2+} \) gives a yellow precipitate of \( \text{CdS} \) on passing hydrogen sulfide (\( \text{H}_2\text{S} \)).

**Explanation**:
- When \( \text{H}_2\text{S} \) is passed through the solution, sulfide ions (\( \text{S}^{2-} \)) are produced.
- \( \text{CdS} \) is known to form a yellow precipitate, while \( \text{NiS} \) forms a black precipitate.
- In a very dilute solution, the concentration of \( \text{Cd}^{2+} \) and \( \text{Ni}^{2+} \) ions are similar, but \( \text{CdS} \) precipitates first due to its lower solubility product (\( K_{sp} \)).

**Conclusion**: The assertion is correct.

### Step 2: Analyze the Reason
The reason states that the solubility product of \( \text{CdS} \) is more than that of \( \text{NiS} \).

**Explanation**:
- The solubility product (\( K_{sp} \)) is a measure of how much of a compound can dissolve in solution. A lower \( K_{sp} \) value indicates that the compound is less soluble and will precipitate out of solution at a lower concentration of ions.
- The experimental values show that \( K_{sp} \) of \( \text{CdS} \) is \( 8 \times 10^{-28} \) and \( K_{sp} \) of \( \text{NiS} \) is \( 3 \times 10^{-19} \).
- Since \( K_{sp} \) of \( \text{CdS} \) is less than that of \( \text{NiS} \), it means \( \text{CdS} \) is less soluble and will precipitate first.

**Conclusion**: The reason is incorrect.

### Final Conclusion
- The assertion is correct: a yellow precipitate of \( \text{CdS} \) forms.
- The reason is incorrect: \( K_{sp} \) of \( \text{CdS} \) is less than that of \( \text{NiS} \).

Thus, the correct answer is that the assertion is correct but the reason is incorrect.

### Answer:
Assertion (A) is correct, Reason (R) is incorrect.