Choose the halogen which is most reactive towards SN1 reaction in...

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 ${Br}_{( a )}$ benzyl carbocation is formed. Benzyl carbocation is more stable than aliphatic carbocation.

By replacing ${I}_{( a )}$ 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 ${Br}_{( a )}$, as carbocation formed at bridge head position is very difficult. Hence, ${Br}_{( b )}$ is more reactive.

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

Hence, the correct answer is option (3).