The electronic configurations of elements $\mathrm{A}, \mathrm{B}$ and $\mathrm{C}$ are $\left[\mathrm{He}\right]2{\mathrm{s}}^1,\left[\mathrm{Ne}\right]3{\mathrm{s}}^1$ and $\left[\mathrm{Ar}\right] {4\mathrm{s}}^1$ respectively. Which one of the following orders is correct for the first ionization potentials of$\mathrm{A}, \mathrm{B}$ and $\mathrm{C}$

All these elements are the succeeding members of a group of the periodic table. It is known that the ionization energy decreases down the group because of increasing the number of shells. 

Hence, the correct order is $\mathrm{A}>\mathrm{B}>\mathrm{C}$.

Therefore, option(a) is the correct answer.