Electrically charged droplet of oil located between two parallel metal surfaces forming the plates of capacitor.
The plate were oriented horizontally, with one plate above the other. Atomised oil drop was introduced through a small hole in between the plates and it is ionised by X-ray, making them negatively charged. First, with zero applied electric field, the velocity of a falling droplet was measured. At terminal velocity, the drag force (viscous force)equals to the gravitational force.
Both forces depends on the radius in different ways, when electric field was applied between the plates then drop were suspended in mechanical equilibrium.
By repeating the experiment for many droplet they confirmed the charges were all small integer multiples of a certain base value which was found to be $1.59 \times 10^{-19} \mathrm{C}$. Magnetic force absent in oil droplet experiment. Thus, option (c) is correct.