Answer
Choice D.
Work Step by Step
The energy of the photon E is goes into ejecting the electron from the surface (the work function of the metal), and the rest goes into kinetic energy of the released electrons.
If a photon has energy of 4 eV and the work function of the metal is 1.5 eV, using photons of 2 eV-- half the energy --will still eject electrons.
However, if the work function were 2.1 eV (i.e., it is more than half of the photon energy), using photons of half the original energy would not eject electrons.