The correct arrangement of decreasing order of atomic radius among Na, K, Mg and Rb is

Na, K and Rb are of group I elements and size increase downthe group 1 elements (↓).Na and Mg are of group 1 and 2 elements respectively but both belong to 3rd period. Along the period size decreases (↓), i.e. size of Na > Mg.Hence the order of sizes is