The element that has the smallest atomic radius is titanium
explanation
Calcium,potassium, scandium and titanium are in period 4 in the periodic table.
Generally the atomic radius decreases across the period (from the left to the right) . This is because across the period there is increase in nuclear charge which make electrons been strongly attracted to the nuclear.
Titanium is in atomic number 22 thus it has more nuclear charge that calcium,potassium and scandium therefore titanium has the smallest atomic radius.
