Choose the halogen which is most reactive towards SN1 reaction in the given compounds (A, B, C & D) A. B. C.…

The leaving group which results in the formation of more stable carbocation will be more reactive towards S_N1 reaction.

In the above molecule, by leaving of ${\mathrm{Br}}_{\left(\mathrm{a}\right)}$ benzyl carbocation is formed. Benzyl carbocation is more stable than aliphatic carbocation.

By replacing ${\mathrm{I}}_{\left(\mathrm{a}\right)}$ in the above molecule, allyl carbocation is formed. Allyl carbocation is more stable than aliphatic carbocation.

In the above molecule, it is difficult to replace ${\mathrm{Br}}_{\left(\mathrm{a}\right)}$, as carbocation formed at bridge head position is very difficult. Hence, ${\mathrm{Br}}_{\left(\mathrm{b}\right)}$ is more reactive.

${\mathrm{Br}}_{\left(\mathrm{a}\right)}$ is more reactive than ${\mathrm{Br}}_{\left(\mathrm{b}\right)}$ in the above molecule, because tertiary carbocation is more stable than primary carbocation.

Hence, the correct answer is option (3).