Choose the halogen which is most reactive towards S_N1 reaction in the given compounds (A, B, C, & D)

Option (1) is correct.

Explanation:

Organic compounds are more reactive towards SN 1, if the carbocation is stable compared to others. Reactivity in reactions depends upon the stability of the carbocation intermediate.

(A)  → Br(a), Becuase formed intermediate carbocation get stable by conjugation with phenyl ring

(B)  → I(a), Becuase formed intermediate carbocation become more stable by conjugation 

(C)  → Br(b), Becuase we can't remove Br(a) from bridge head carbon (Bredt's rule

(D)  → Br(a), because formed intermediate 3° carbocation is more stable (stability of carbocation 3° > 2° > 1°)