Assertion `(A:` Cell constant is the `EMF` of a cell.
Reason `(R):` Cell constant is determined by using saturated `KCl` solution.

Options

A. If both `(A)` and `(R)` are correct, 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, `(R)` is correct.

Explanation

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states: "Cell constant is the EMF of a cell."
- The cell constant (denoted as \( G \)) is defined as \( G = \frac{l}{A} \), where \( l \) is the distance between the electrodes and \( A \) is the surface area of the electrodes.
- The EMF (Electromotive Force) of a cell is the maximum potential difference between the two electrodes, which is not the same as the cell constant.
- Therefore, the assertion is **incorrect**.

2. **Understanding the Reason**:
- The reason states: "Cell constant is determined by using saturated KCl solution."
- The cell constant can indeed be determined using a solution of saturated KCl, as KCl has a known conductivity that can be used to measure the resistance of the cell.
- The conductivity of the KCl solution allows us to calculate the cell constant accurately.
- Therefore, the reason is **correct**.

3. **Conclusion**:
- Since the assertion is incorrect and the reason is correct, we can conclude that the correct answer is option 4: "Assertion is incorrect and reason is correct."

### Final Answer:
- Assertion (A) is incorrect.
- Reason (R) is correct.
- Therefore, the correct option is: **4. Assertion is incorrect and reason is correct.**