The species with largest ionisation potential

Atomic number of Li and its electronic configuration is 1s² 2s¹. When Li becomes ${\mathrm{Li}}^{+}$, then it achieves a noble gas configuration, 1s². Now, it is difficult to remove an electron from ${\mathrm{Li}}^{+}$. So, ${\mathrm{Li}}^{+}$ has the largest ionization potential.

Hence, option (a) is correct.