When atoms gain electrons they have more electrons, so they increase their size due to the energy levels, orbitals, etc. Those are the atoms that form negative ions. (Non Metals)
When atoms loose electrons and form positive ions they loose components and energy levels. Therefore they decrease their size.
The only one here who forms negative ions is Bromine (Br) that forms Bromide (Br-1)
So it will be answer B) Br