Question 18
Mg2+ is the answer
Only cations will have lost electrons leading to a fall in ionic radius.
However, anions will gain electrons leading to an increase in ionic radius.
Thus we eliminate A and D.
Now we must choose between Mg and Na.
Mg has more protons than Na. Hence, Mg's nucleus has a greater positive force of attraction which works on its. electrons
When it loses its electrons this RELATIVELY larger force of attraction will attract the outer electrons inwards more than Na can. This will shrink the ionic radius EVEN MORE than Sodium.