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 =  $13.6\left(1-\frac{1}{2^2}\right)eV=10.2eV$

$\therefore {\left(K.E\right)}_{\max }=\left(15-10.2\right)eV=4.8eV$