The correct arrangement of the increasing order of atomic radius among Na, K, Mg, and Rb is:

The correct order of increasing order of atomic radius among Na, K, Mg, and Rb is Mg < Na < K < Rb.
“The distance from the centre of the nucleus to the outermost shell of electrons in an atom is called atomic radius”.

On moving down a group, a new shell of electrons is added at each succeeding element. This increases the nuclear charge of elements. This produces a contractive effect on the atomic radius. The increase in the atomic radius due to the addition of new shells is so large that it outweighs the contractive effect of the increased nuclear charge.

As a result, the atomic radius increases on moving down a group from top to bottom and the nuclear charge decreases. Mg is smaller in size than Na, as it lies on the right side of Na. K, Na, and Rb belong to the same group and Rb has the largest size. Na comes above K so has a smaller size than K.