Don’t currently have a calculator with me but just use Avogadros constant (A) 6.02x10^24, in the equation n=N/A. Lower case n being the number of mol and upper case being the number of molecules (given).
The answer would be letter A - bromine. Bromine <span>(B<span>r2</span>)</span>
has only london dispersion forces and are weak, but it's large electron
cloud allows for transient polarization creating dipole moments that
strengthen their interaction.