The complex $\left[\mathrm{Cr}{\left({\mathrm{H}}_2\mathrm{O}\right)}_4{\mathrm{Br}}_2\right]\mathrm{Cl}$gives the test for :

Since ${\mathrm{Cl}}^{-}$ is free in  $\left[\mathrm{Cr}{\left({\mathrm{H}}_2\mathrm{O}\right)}_4{\mathrm{Br}}_2\right]\mathrm{Cl}$ complex it gives the test for ${\mathrm{Cl}}^{-}$.

${\mathrm{Cl}}^{-}$ is free because it is not part of the coordination sphere, which causes the complex.

Hence correct option is B.