Question

The smallest unit of charge is –1.6× 10–19 C, which is the charge (in coulombs) of a single electron. Robert Millikan was able to measure the charge on small droplets of oil by suspending them between a pair of electrically charged plates. Which of the following are possible charges of those oil droplets?

Answer 1

Any number that is a multiple of 1.6 is correct, because you can't have a fraction of an electron. Obviously the first one is wrong, because it's charge is smaller than the charge of a single electron, implying that it has a fraction of an electron.
( 1) -3.2x10^-19C
(2 ) -4.8x10^-19C