In 1909, Robert Millikan and Harvey Fletcher conducted the oil drop experiment to determine the charge of an electron. They suspended tiny charged droplets of oil between two metal electrodes by balancing downward gravitational force with upward drag and electric forces.

Charge on the electron was determined by:

a) R.A. Millikan's oil drop experiment

R.A. Millikan's oil drop experiment is the correct answer for determining the charge on the electron. This groundbreaking experiment, conducted in 1909, provided the first direct measurement of the charge of an electron. Millikan's experiment involved observing the motion of charged oil droplets in an electric field.

Here is a detailed explanation of the experiment:

1. Principle of the experiment:
- Millikan used a specially designed apparatus that consisted of a chamber filled with air, where he introduced tiny oil droplets.
- The oil droplets were charged by adding or removing electrons using X-rays or ionizing radiation.
- An electric field was applied to the chamber, which allowed the droplets to be suspended in mid-air.

2. Measurement of the charge:
- By adjusting the strength of the electric field, Millikan was able to observe the motion of the charged droplets.
- He found that some droplets remained stationary, while others moved up or down.
- By carefully adjusting the electric field, he could balance the gravitational force acting on the droplets with the electric force, causing them to remain suspended.
- By measuring the strength of the electric field required to balance the forces, Millikan could determine the charge on the droplets.

3. Analysis of the results:
- Millikan performed the experiment with various droplets and observed that the charge on all the droplets was always an integral multiple of a fundamental unit of charge.
- This fundamental unit of charge was found to be the charge of a single electron.
- Millikan's measurements determined the charge on the electron to be approximately -1.6 x 10^-19 Coulombs, which is the charge we now accept as the fundamental charge of an electron.

Conclusion:
Based on R.A. Millikan's oil drop experiment, the charge on the electron was determined to be -1.6 x 10^-19 Coulombs. This experiment provided essential evidence for the existence and properties of electrons, contributing significantly to our understanding of atomic structure and electrodynamics.