Ionization energy is highest for

Explain the trend observed in ionisation enthalpy when moving from top to bottom in the same group of the periodic table.

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:

What is the decreasing order of the second ionization enthalpy for $Mg, Ca,$ and $Ba$?

The first ionisation potential is maximum for

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