The ionization energy of nitrogen is larger than that of oxygen because of

Which one of the following orders is correct for the first ionisation energies of the elements?

Ionization energy of gaseous $Na$ atoms is $495.5 \ kJ \ mol^{-1}$. The lowest possible frequency of light that ionizes a sodium atom is $(h = 6.626 \times 10^{-34} \ J \ s, N_A = 6.022 \times 10^{23} \ mol^{-1})$.

For one of the elements,various successive ionization enthalpies (in $kJ \cdot mol^{-1}$) are given below:

$I.E.$	$1^{st}$	$2^{nd}$	$3^{rd}$	$4^{th}$	$5^{th}$
Value	$577.5$	$1810$	$2750$	$11580$	$14820$

The element is:

How would you explain the fact that the first ionization enthalpy of sodium is lower than that of magnesium but its second ionization enthalpy is higher than that of magnesium?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ The energy required to form $Mg^{2+}$ from $Mg$ is much higher than that required to produce $Mg^{+}$.
Reason $R:$ $Mg^{2+}$ is a small ion and carries more charge than $Mg^{+}$.
In the light of the above statements,choose the correct answer from the options given below: