$\begin{array}{1 1} \text{(a) The radiation is sufficiently intense.} \\\text{(b) The work function of the plate is less than the energy of a single photon and the plate is uncharged.} \\ \text{(c) The wavelength of the radiation exceeds a minimum value.} \\ \text{(d) The plate is freshly cleaned. } \end{array}$
