The electronic structure of four elements a, b, c and d are 

a) ${\text{1s}}^{\text{2}}$          b) ${\text{1s}}^{\text{2}}{\text{,\hspace{0.17em}2s}}^{\text{2}}{\text{,\hspace{0.17em}2p}}^{\text{2}}$          c) ${\text{1s}}^{\text{2}}{\text{,\hspace{0.17em}2s}}^{\text{2}}{\text{,2p}}^{\text{5}}$          d) ${\text{1s}}^{\text{2}}{\text{,\hspace{0.17em}2s}}^{\text{2}}{\text{,\hspace{0.17em}2p}}^{\text{6}}$

The tendency to form electrovalent bond is greatest in