The electronic configurations of elements A,B and C are [He]2s¹, [Ne] 3s¹ and [Ar] 4s¹ respectively. Which one of the following order is correct for the first ionisation potentials of A, B and C

As we can see that all these elements are the succeeding members of a group of periodic table.

And we know that ionisation energy decreases with increase in number of shells.same will happen if we move down the group,

We can say that as we move down the group ionisation energy decreases.

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

Therefore, option(a) is the correct answer.