The correct order of effective nuclear charge ${\mathrm{Z}}_{\mathrm{eff}}$ is :

It will be simpler to add an electron to a smaller atom because the new electron will often be closer to the positively charged nucleus because the effective nuclear charge grows from left to right during a period. Therefore, the proper sequence is $\begin{array}{r}\mathrm{B}<\mathrm{C}<\mathrm{N}<\mathrm{O}<\mathrm{F}\end{array}$.

Thus, the option A is correct.