The energy required to remove the second electron after the first electron has been removed can be calculated using the formula for the Bohr energies.
$\begin{align*}E_1 = 4 \cdot 13.6 \mbox{ eV} = 54.4 \mbox{ eV}\end{align*}$
Therefore, the energy required to remove the first electron is
$\begin{align*}79.0 \mbox{ eV} - 54.4 \mbox{ eV} = \boxed{24.6 \mbox{ eV}}\end{align*}$
Therefore, answer (A) is correct.

Solution to 2001 Problem 18