Answer is: 3) francium.
The atomic radius of a chemical element is a measure of the size of its atom.
The atomic radius varies with increasing atomic number, but usually increases because of increasing of number of electrons.
Potassium (K), rubidium (Rb), ceasium (Cs) and francium (Fr) are in group 1 (the alkali metals) of the periodic tabl.
The atomic radius increases down the group of Periodic table and francium is on the bottom of the group 1 with highest atomic number.
