The atomic radius identifies the average distance between two nuclei of the same element bonded together.
In the periodic table, the larger the group to which the element belongs, the smaller the atomic radius. The groups of the periodic table refer to the columns. So in the first column is group IA and in the last column is group VIIIA.
We are going to classify the elements we are given by the group they belong to and so we can order them according to their increasing atomic radius.
Potassium ---> IA
Calcium--->IIA
Bromine --->VIIA
Arsenic--->VA