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

Einstein realized that the energy in electrons was caused by the frequency of light and not the intensity of light. This made him realize that he needed to explain the photoelectric effect.

Explanation:

Einstein realized that the photoelectric effect was a system that should be studied and explained in more depth, when he saw how the energy of electrons behaved in the presence of light. He saw that this energy depends entirely on the frequency of light and not on the intensity of light. From this observation, he realized that the photoelectric effect was essential for the production of an electric energy system, through light energy, such as solar energy.

It is important to note that the photoelectric effect refers to the ejection of electrons on a light surface that comes into contact with a light source.