Identify the correct order of reactivity for the following pairs towards the respective mechanism (A) (B)…

The rate of ${\mathrm{S}}_{\mathrm{N}}2$ reaction decreases with increase in steric crowding. In the given set of molecules the second molecule(Tertiary alkyl halide) is sterically crowded, hence, it is correct option. More stable carbocation formed, higher will be the reactivity of ${\mathrm{S}}_{\mathrm{N}}1$. Benzyl carbocation is more reactive than aliphatic carbocation.

The deactivating groups decreases the rate of electrophilic substitution reaction. ${\mathrm{NO}}_2$ is strong deactivating group for electrophilic substitution.

In the aromatic nucleophilic substitution reactions, the intermediate formed is carbanion and, it is stabilised by ${\mathrm{NO}}_2$ at ortho or para positions.

Hence $\mathrm{A}, \mathrm{B}, \mathrm{C}, \mathrm{D}$ all are correct.