Answer:
A. Br
Step-by-step explanation:
Atomic radius increases down the group. As the increase in the attractive force of the nucleus increases owing to increase in the size of its positive charges, extra electrons are added to the shells to more than counterbalance this effect down the group hence the atomic radius becomes larger. Iodine has a larger atomic radius than bromine.
Secondly, the ionic radius of a negative ion is greater than its corresponding atomic radius since a negative ion is formed by adding a negative charge (electron) to the atom thus making it bigger. Hence the radius of I^- is greater than that of Br^-.
This means that Br atom must possess the smallest radius among the options listed.