Which observation confused scientists regarding the photoelectric effect? a. low-intensity light does not always cause electrons to be emitted. b. high-intensity light does not always cause electrons to be emitted. c. low-frequency light does not always cause electrons to be emitted. d. high-frequency light does not always cause electrons to be emitted

Light does not always cause electrons to be emitted.
If it doesn't, then you might think that making the light brighter
would cause electrons to be emitted. That DOESN't work, but
making the light higher FREQUENCY causes electrons to be
emitted !

Did you catch that ? ==> If you light a match next to a piece of metal, and the match burns with a red flame and NO electrons are emitted from the metal, then you can move in with a military searchlight or a ginormous high-power laser that has the same red color and shine it on the metal, and still no electrons will be emitted. But if you light a little match next to the metal and the match burns with a BLUE flame, then electrons WILL be emitted. Knocking electrons out of the metal has nothing to do with how strong and bright the light is. It only depends on the FREQUENCY (color) of the light !

This observation was confusing until the beginning of the 20th Century,
even to the smartest scientists. There was no way to explain it until 1905,
when Albert Einstein pretty much invented Quantum Mechanics to explain
this "Photoelectric Effect".

Of all of Einstein's great discoveries, THIS was the one for which he was
awarded the Nobel Prize in Physics ... in 1922. He never won a Nobel for
his theories of Relativity.