Which one of the following elements has the highest ionisation energy?

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:

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?

The first ionization enthalpy of $O$ is less than that of $N$,because .........

The correct order of first ionisation enthalpy of the given elements is:

What may be the first ionisation energy of $Na$ if it has an effective nuclear charge of $1.84$,assuming the Bohr model of the $H$-atom is effective on it? (in $eV$)