The correct arrangement of increasing order of atomic radius among Na, K, Mg, Rb is:
Mg < Na < K < Rb
Mg < Na < K < Rb. The order is based on the trend in a period and a group. The atomic radius decreases across a period (from left to right).
The atomic radius increases down a group due to the addition of a new energy level. Mg lies on the right side of Na and so it is smaller in size than that of Na.
K lies below Na in the same group and so, the size of K is greater than that of Na. And similarly, Rb is in the same group as K, Na and lies below K, Na.
Hence it has the largest size.