Photoelectric effects involve the emission of electrons from the metal surface due to the incidence of light radiations.
Electrons are easily knocked out of atoms when ionization energy (I.E.) is low.
I.E. is low when there is high shielding and low effective nuclear charge. Therefore, I.E. decreases down the group.
Thus, in the given options, the element which exhibit photoelectric effect with light of the longest wavelength is Rb.