Question

What's threshold energy must photon have to produce a photoelectron from aluminum which has work function of 4.20 ev?

Answer 1

Threshold energy is 4.20 eV, because work function is the minimum energy required to remove an electron from the metal surface.

if you learnt about photoelectric effect, if photon energy strikes to metal surface , electrons of metal surface eject from it and get some energy in form of kinetic energy.
e.g., $K.E=h\nu-\Phi$
here $\Phi$ is thresold energy, it is minimum energy required to remove an electron from metal surface.