The first  $\left({\mathrm{IE}}_1\right)$and second $\left({\mathrm{IE}}_2\right)$ionisation energies (kJ/mol) of a few elements designated by Roman numerals are given below. Which of these would be an alkali metal? 

As outermost electronic configuration of alkali metals is ns¹ and also their size are largest in their respective periods so their 1st IE will be lowest among the given options. As second electron is to be removed from complete shell or noble gas core, so the 2^nd I.E. must be highest among the given options. So, option (b) is correct choice.