Correct order of reactivity towards nucleophilic substitution reaction of following is :-

Identify $\mathrm{A}$ and predict the type of reaction|:  NEET (UG) 2017

For the following:

(A) ${\mathrm{I}}^{-}$      (B) ${\mathrm{Cl}}^{-}$       (C) ${\mathrm{Br}}^{-}$

The increasing order of nucleophilicity would be

In a ${\mathrm{S}}_{\mathrm{N}}2$ substitution reaction of the type

$\mathrm{R}-\mathrm{Br}+{\mathrm{Cl}}^{-}\stackrel{\mathrm{DMF}}{⟶}\mathrm{R}-\mathrm{Cl}+{\mathrm{Br}}^{-}$ 

which one of the following has the highest relative rate ?