Which observation provided Albert Einstein the clue that he needed to explain the photoelectric effect?

The key observation was that the energy of emitted electrons changes with the light’s frequency, not its intensity. Einstein explained this by proposing that light transfers energy in packets: higher-frequency light has more energetic packets, while increasing intensity mainly adds more packets. So the correct choice is “Energy of electrons depends on light’s frequency, not intensity.”

AI-written and checked against the verified answer.