Correct order of first I.P among following elements

 Be, B, C, N, O 
 

$\mathrm{Be}-1{\mathrm{s}}^{2}2{\mathrm{s}}^2;\mathrm{B}-1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^1;\mathrm{C}-1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^2;\mathrm{N}-1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{3;}\mathrm{O}-1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^4$.

 IP increases along the period. But IP of Be > B. Further IP of O < N because atoms with fully or partly filled orbitals are most stable and hence have high ionisation energy.