Concept:Moving down a group, each element gains an extra electron shell, which increases the overall size of its atoms and ions.
Explanation:Down a group of the periodic table, a new principal energy level or electron shell is added.
This added shell increases the distance between the nucleus and the outermost electrons.
Since the outermost electrons are farther from the nucleus, the atomic radius becomes larger.
For ions formed from these atoms, the ionic radius also increases down the group for the same reason.
Now look at the other options: electronegativity generally decreases down a group.
Electron affinity also tends to become less negative down a group.
Ionization energy decreases down a group because it is easier to remove an outer electron.
Therefore, the property that increases down a group is ionic radius.
Answer:C. ionic radius