Among the following,which has the highest first ionization energy?

Ionisation energy in group $I$ elements varies in the decreasing order as:

The given electronic configurations are for elements $X$,$Y$,and $Z$. Note that these configurations represent ions of the elements:
$X = [Ne] \, 3s^2 \, 3p^5$
$Y = [Ne] \, 3s^2 \, 3p^6$
$Z = [Ne] \, 3s^2 \, 3p^4$
Determine the correct statement regarding the energy changes associated with these configurations.

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

What is ionization enthalpy? What is the unit of ionization enthalpy?

Which of the following has the highest first ionisation energy?