Which element has the smallest atomic radius? calcium potassium scandium titanium

 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.