In periodic table, the size of atom decreases from left to right. which means $F$ is smaller than $O$ and $N$.
Also, a free atom has smaller size in comparision to it's anion because the excess electron increases the repulsion amory the electrons and increases in size. So, $F$ is smaller than $F^-$
$(a)\; C \;\;(b)\; O \;\;(c)\; F\;\; (d) \;Cl\;\; (e)\; Br$