The two compounds $\left[\mathrm{Cr}{\left({\mathrm{NH}}_3\right)}_5\mathrm{Br}\right]\mathrm{Cl}$ and $\left[\mathrm{Cr}{\left({\mathrm{CN}}_3\right)}_5\mathrm{Cl}\right]\mathrm{Br}$ can be distinguished by reagent A and the two compounds exhibit isomerism (B). Then (A) and (B) are

 (A) and (B) are $\left[\mathrm{Cr}{\left({\mathrm{NH}}_3\right)}_5\mathrm{Br}\right]\mathrm{Cl}$ and $\left[\mathrm{Cr}{\left({\mathrm{NH}}_3\right)}_5\mathrm{Cl}\right]\mathrm{Br}$ can be distinguished by ${\mathrm{AgNO}}_3$ and isomerism shown is ionisation isomerism.
${\mathrm{AgNO}}_3$ reacts with ${\mathrm{Cl}}^{-}$creating a white precipitate of an atom in the outer coordination sphere of $\mathrm{AgCl}$. When ${\mathrm{Br}}^{-}$gives a light yellow precipitation in the outer coordination sphere, $\mathrm{AgBr}$ forms.