There is no need to work out the
frequency.
If the photons have an energy of 5.0 eV and the work function of the
surface is 3.5 V it means that the maximum energy of the photoelectrons is 1.5 eV. 3.5 eV
will be used in 'bringing' the free electrons to the surface of the metal leaving a maximum of
1.5 eV for kinetic energy.
It is a maximum because energy may also be liberated as
heat.
The answer is (a)