The property of elements which increases down a group of the periodic table is

Step 1: Analyze the question We need the property that increases as we go down a group (for example, from Li to Na to K).

Step 2: Concept: trends down a group Going down a group, each element gains an extra electron shell. This has these effects:

• The atomic and ionic radii increase, because there are more shells.
• The outer electrons sit farther from the nucleus and are better shielded by inner electrons.
• The nuclear attraction on the outer electrons therefore weakens.

Step 3: Check each option

• A. Electronegativity: decreases down a group, because the atom's ability to attract electrons weakens.
• B. Electron affinity: generally decreases (becomes less negative) down a group, because the incoming electron is farther from the nucleus.
• C. Ionic radius: increases down a group, because more shells are added (Li⁺ < Na⁺ < K⁺).
• D. Ionization energy: decreases down a group, because outer electrons are easier to remove.

Step 4: Answer Only ionic radius increases down a group.

Tip: Remember that going down a group, size increases while everything related to attracting or holding electrons (electronegativity, ionization energy, electron affinity) decreases. Across a period, the pattern is the opposite: those properties increase and size decreases.