If the ionisation energy of hydrogen is $2.18\times {10}^{-19}J$, the first ionisation energy of ${He}^{+}$ion is:

The correct option is C $8.72\times {10}^{-19}J$

The energy of an electron in the first orbit $=-2.18\times {10}^{19}J$

The energy of an electron in the fifth orbit

$=\frac{-2.18\times {10}^{-19}}{n_2}$

$=\frac{-2.18\times {10}^{19}}{25}$

$E_5=8.72\times {10}^{-19}$ $J$

Hence, option $C$ is correct.