A photon of energy 4 eV is incident on a metal surface whose work function is 2 eV. The minimum reverse potential to be applied for stopping the emission of electrons is

Question

$\begin {array} {1 1} (a)\;2\: V & \quad (b)\;4\: V \\ (c)\;6\: V & \quad (d)\;8\: V \end {array}$