Potassium belongs to group - (1) and period - (iii) of the periodic table. Hence among the given options Potassium has a maximum atomic radius.
Carbon and Nitrogen belong to period - (ii) of the periodic table and on the right side hence it is obvious that atomic radii of Carbon and Nitrogen are very much small than Sodium and Potassium.
Sodium and Potassium belongs to group - (1) and Potassium is placed below Sodium hence atomic radius of Potassium more than Sodium
Important Point
The atomic size generally decreases across a period as we move from left to right in a particular period.
The atomic size generally increases across group as we move from top to bottom in a particular group.