Molecular orbital electronic configuration for ‘X ’ anion is

$\mathrm{KK}\left(\mathrm{\sigma }2\mathrm{s}{)}^2\right(\mathrm{a}2\mathrm{s}{)}^2{\left(\mathrm{\pi }2{\mathrm{p}}_{\mathrm{x}}\right)}^2{\left(\mathrm{\pi }2{\mathrm{p}}_{\mathrm{y}}\right)}^2{\left(\mathrm{a}2{\mathrm{p}}_{\mathrm{z}}\right)}^2{\left(\mathrm{\pi }2{\mathrm{p}}_{\mathrm{x}}\right\}}^{\ast }$

The anion ‘X ’ is 

Given electronic configuration of anion X is 

∴ Total number of electrons of anion X = 15
Hence, the anion X is N₂^-.