The order of reactivity of halides towards SN1 mechanism is:

The order of reactivity of SN1 reactions depends on the stability of carbocation intermediate formed during nucleophilic substitution reaction. So pi bonds and bulky alkyl groups increases the stability of carbocations. So the correct order of stability is Benzyl > Allyl > 3o > 2o > 1o > methyl. Hence option D is correct.