Explanation:

- J.J. Thomson - Discovered the electron in 1897 through cathode ray experiments. He identified the electron as a negatively charged particle, which was a groundbreaking revelation at the time.

- Rutherford - Known for discovering the nucleus in atoms through the gold foil experiment. This suggested a small, dense core within atoms but did not involve the electron discovery.

- Moseley - Known for his work on X-ray spectra, which led to the discovery of Moseley's law in X-ray spectroscopy; he didn't discover the electron.

- Millikan - Famous for the oil-drop experiment, which measured the charge of the electron. He clarified properties of the electron rather than discovering it.

The correct option is: Option 3: Thomson

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