Ba Cr F He
A. Ne, Br, Ca, Ba
Ionization energy is the minimum amount of energy required to remove n electron from an atom.
The general trend is increase in moving from left to right across periods in the periodic table and decreasing down the group.
Barium, Ba belongs to period 6, so it is expected to have the lowest ionization energy.
Neon, Ne belongs to period 2, it would have the highest ionization energy.
Calcium, Ca and Bromine, Br both belongs to the same period, however bromine is in group 7, bromine would have the higher ionization energy.
Inorder of decreasing ionzation enrgy;
Ne, Br, Ca, Ba