The energy of electrons in the K-shell is least for

Among all ($\mathrm{H}$,${\mathrm{He}}^{+}$,${\mathrm{Li}}^{+2}$ and ${\mathrm{Be}}^{+3}$). In a Be⁺³ ion, there is only one electron remaining, and it will occupy the K-shell, which is the closest shell to the nucleus. The effective nuclear charge experienced by this electron is greater because there are fewer electrons to shield it from the positive charge of the nucleus. As a result, this electron is more strongly attracted to the nucleus and has less energy compared to electrons in atoms or ions with more electrons in their K-shell.