c. It decreases when going down a group.
Explanation:
Ionization energy is the energy needed to remove an electron from the neutral gaseous atom.
It is represented in kJ/mol.
As we move down the group the ionization energy decreases, since the outermost electrons are far away from the nucleus of an atom and so attains greater shielding effect. They encounter a weaker force of attraction to the +ve charge on the nucleus. And so, Ionization energy decreases from top to bottom in a group.
