A photon of energy 15 eV collides with a stationary H-atom. Due to this collision, H-atom gets ionized. The maximum kinetic energy of emitted electron is

If the H-atom does not move after emission of electron, the electron will be ejected with maximum K.E. Minimum energy absorbed by the  H-atom =  13.61−122eV=10.2 eV∴   K.Emax  =  15−10.2eV=4.8eV