Answer:
A. K
Explanation:
Radius of an atom is directly propotional to number of shells. Potassium (K) has 4 shells (as it is in period 4) whlie sodium (Na) has 3 shells (as it is in period 3) therfore K > Na.
Cations are always smaller than their parent atoms. so increasing order is
Na+ < Na < K+ < K
