Assertion: - Higher the `R_f` value of a substance, greater is the time taken by it for elution.
Reason :- The time taken by a substance for elution is greater when it is adsorbed more strongly on the adsorbent.

Options

A. If both Assertion and Reason are CORRECT and Reason is the CORRECT explanation of the Assertion.
B. If both Assertion and Reason are CORRECT but Reason is not the CORRECT explanation of the Assertion.
C. If Assertion is CORRECT but Reason is INCORRECT.
D. If Assertion is INCORRECT but Reason is CORRECT.

Explanation

To solve the question, we need to analyze the assertion and the reason provided in the context of chromatography, specifically focusing on the relationship between the \( R_f \) value and the time taken for elution.

### Step-by-Step Solution:

1. **Understanding \( R_f \) Value**:
- The \( R_f \) value (retention factor) is defined as the ratio of the distance traveled by the compound to the distance traveled by the solvent front in a chromatographic process.
- Mathematically, it is given by:
\[ R_f = \frac{\text{Distance traveled by the substance}}{\text{Distance traveled by the mobile phase}} \]

2. **Analyzing the Assertion**:
- The assertion states: "Higher the \( R_f \) value of a substance, greater is the time taken by it for elution."
- This statement is incorrect. A higher \( R_f \) value indicates that the substance has traveled further relative to the solvent front, which typically implies weaker interactions with the stationary phase (adsorbent). Therefore, it should take less time for elution, not more.

3. **Analyzing the Reason**:
- The reason states: "The time taken by a substance for elution is greater when it is adsorbed more strongly on the adsorbent."
- This statement is correct. When a substance is adsorbed more strongly to the stationary phase, it moves more slowly through the column, resulting in a longer elution time.

4. **Conclusion**:
- Since the assertion is incorrect and the reason is correct, we conclude that the correct option is:
- Assertion: Incorrect
- Reason: Correct

### Final Answer:
- The assertion is incorrect, while the reason is correct.