The energy of the electron in the hydrogen atom is given by the expression:

Question

\[\begin {array} {1 1} (a)\;\frac{-e^2}{r^2} & \quad  (b)\;\frac{-n^2h^2}{2 \pi 2z^2 e^4m} \\  (c)\;\frac{-2\pi^2 z^2e^4}{n^2h^2} & \quad (d)\;\frac{nh}{2 \pi} \end {array}\]