From the following elements,which of them has the highest second ionisation potential?

Why does the successive ionization enthalpy of an element increase in the order: $1^{st} < 2^{nd} < 3^{rd}$?

Which of the following represents the correct decreasing order of the first ionization enthalpy of $Ba, Sr, Ca,$ and $Mg$?

Correct increasing order of first ionisation potential is

Assertion $(A)$: First ionisation enthalpy of oxygen is less than that of nitrogen.
Reason $(R)$: Atoms with half-filled or completely filled orbitals are less stable.
The correct option among the following is:

$B$ has a smaller first ionization enthalpy than $Be$. Consider the following statements:
$(I)$ It is easier to remove a $2p$ electron than a $2s$ electron.
$(II)$ The $2p$ electron of $B$ is more shielded from the nucleus by the inner core of electrons than the $2s$ electrons of $Be$.
$(III)$ The $2s$ electron has more penetration power than the $2p$ electron.
$(IV)$ The atomic radius of $B$ is more than $Be$.
(Atomic number: $B=5, Be=4$)
The correct statements are: