Based on the periodic table, atomic size increases from top to bottom within a group. This is because, electrons are added to new outer shells. These outer electrons are shielded by the inner electrons and therefore experience a lower nuclear attraction which results in an increase in the atomic size (radius).
Now, Mg precedes Br in the periodic table, i.e. Mg belongs to the 3rd period while Br belongs to the 4th. Thus, Br will have more orbitals that are occupied by electrons and hence a larger size.
Ans: B)
