What element out of K K plus Rb Rn plus has the smallest radius?

The element with the smallest atomic radius among K (potassium), Rb (rubidium), and Rn (radon) is Rn (radon). As you move down a group in the Periodic Table, atomic radii increase due to the addition of electron shells. However, radon is a noble gas with a completely filled outer shell, which leads to a relatively smaller effective radius compared to the alkali metals K and Rb. Therefore, Rn has the smallest radius among the given elements.