Which experiment is responsible for finding out the charge on an electron?
a.) – Millikan’s oil drop experiment
b.) – Cathode ray discharge tube experiment
c.) – Rutherford’s $\alpha$–rays scattering experiment
d.) – Photoelectric experiment

Hint: In the options, we are given with the four experiments performed in the chemistry. These all are relatable to the electrons like charge, sign, irradiation on the metal surface, presence of electrons around the nucleus. Identify the experiment related to the charge on an electron.

Complete step by step solution:
Now, we will discuss all the experiments one by one.
The first we have is Millikan’s oil drop experiment. It was first performed by Robert A. Millikan.

From the name itself, we can say it is related to the oil drops. In this experiment, by applying the electric field, the tiny droplets of oil were allowed to pass through the hole.
So, this experiment led to the calculation of the charge over the oil droplet.
The second we have Cathode ray discharge tube; it was performed by Karl Ferdinand Braun.
In this experiment, we can see that the result is produced by applied electric, or magnetic field. This leads to the indication of sign of electron charge i.e. positive, or negative.
Talking about the third we have Rutherford’s $\alpha$–rays scattering experiment. It was performed by the J.J Thomson model.
We can say that this experiment is defined in terms of the plum pudding model. This experiment led to the conclusion that there is presence of electrons around the nucleus.
The fourth we have is a Photoelectric experiment. In this experiment, we have the ejection of electrons known as photoelectrons, when there is ejection of light on the metal surface.
So, we can say that experiment tells about the light irradiation on the metal surface.
In the last, we can conclude that Millikan’s oil drop experiment is responsible for finding out the charge on an electron.

Hence, the correct option is (A).

Note: Don’t get confused while determining the experiment performed for finding the charge on an electron. We must know about the observational data obtained in the experiments. On the basis of that, we have determined the concluding answer.