Assertion : In Millikan's experiment for the determination of charge on an electron, oil drops of any size can be used.
Reason : Millikan's experiment determines the charge on electron, by simply measuring the terminal velocity.

Options

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

Explanation

To solve the given assertion and reason question, we need to analyze both the assertion and the reason independently and then determine their validity.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "In Millikan's experiment for the determination of charge on an electron, oil drops of any size can be used."
- In Millikan's oil drop experiment, it is crucial to use oil drops that are of microscopic size. This is because larger drops would require an impractically high amount of electric charge to balance their weight in the electric field. Therefore, the assertion is **false**.

2. **Understanding the Reason**:
- The reason states that "Millikan's experiment determines the charge on an electron by simply measuring the terminal velocity."
- This statement is true. Millikan's experiment involves balancing the gravitational force acting on the oil drop with the electric force acting on it, which can be derived from the terminal velocity of the drop. The charge on the electron is calculated using the relationship between these forces.

3. **Analyzing the Relationship**:
- The relationship involves the formula for the terminal velocity of a spherical drop in a viscous medium, which is given by:
\[ v_t = \frac{2r^2(\rho - \sigma)g}{9\eta} \]
- Here, \( v_t \) is the terminal velocity, \( r \) is the radius of the drop, \( \rho \) is the density of the oil, \( \sigma \) is the density of air, \( g \) is the acceleration due to gravity, and \( \eta \) is the viscosity of the fluid. The charge on the electron can be derived from the balance of forces when the drop is suspended in an electric field.

4. **Conclusion**:
- Since the assertion is false and the reason is true, the correct conclusion is that the assertion does not support the reason. Thus, the answer to the question is that the assertion is false, and the reason is true.

### Final Answer:
- The assertion is **false** and the reason is **true**.