In which alkyl halide,$SN^2$ mechanism is favoured maximum?

In a $S_N2$ substitution reaction of the type $R-Br + Cl^- \xrightarrow{DMF} R-Cl + Br^-$,which one of the following has the highest relative rate?

Which among the following is most reactive via $S_N2$ mechanism?

Assertion $A:$ Hydrolysis of an alkyl chloride is a slow reaction but in the presence of $NaI$,the rate of the hydrolysis increases.
Reason $R:$ $I^{-}$ is a good nucleophile as well as a good leaving group.
In the light of the above statements,choose the correct answer from the options given below.

The order of reactivity of the compounds $C_6H_5CH_2Br$,$C_6H_5CH(C_6H_5)Br$,$C_6H_5CH(CH_3)Br$ and $C_6H_5C(CH_3)(C_6H_5)Br$ in $S_N2$ reaction is

Arrange the following compounds in order of decreasing rate of hydrolysis for $S_N1$ reaction:
$(i)$ $C_6H_5CH_2Br$
(ii) $p-CH_3-C_6H_4-CH_2Br$
(iii) $p-CH_3CH_2-C_6H_4-CH_2Br$
(iv) $p-(CH_3)_2CH-C_6H_4-CH_2Br$