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 electrons by simply measuring the terminal velocity.
A. Both assertion and reason are correct and reason is the correct explanation for assertion.
B. Both assertion and reason are correct but reason is not the correct explanation for assertion.
C. Assertion is correct but Reason is incorrect.
D. Assertion is incorrect but the reason is correct.
E. Both assertion and reason are incorrect.

Hint: We are well known for this Millikan’s oil drop experiment, it is the first ever thing we have considered to be true in accordance with the presence of particles in the matter. Thus, here we have to recall the concept that we have learnt in the Millikan experiment and solve this particular question.

Complete step by step solution:
First of all let us talk about assertion. In Millikan’s oil drop experiment it is determined to use microscopic sizes of drops because the results cannot be drawn if we use a large size of drops. So the assertion is incorrect. We cannot use any size of the oil drops rather than microscopic size.

Now, reason says that experiment determines the charge on electron, by measuring the terminal velocity which is true for that we have to look into the comparison of the mathematical instance of oil drop density and air density such that:
\[\dfrac{4}{3}\pi {a^3}g\left( {{\rho _{liquid}} - {\sigma _{air}}} \right) = 6\pi a\eta v\]
In this formula, if \[a\] is large then \[v\] is also large which can cause errors. So, the assertion is incorrect and the reason is correct.

Hence, the correct answer is option D.

Note: This question is totally memory based, that means it is based on the basics of structure of atoms and electric charges, when we start learning about matter we come across such experiments and have to recall them from time to time.