Here in case of (1) it is more favorable towards SN1 reaction as it will create benzyl carbocation which is very stable.
(2) Here it is more favorable towards SN1 as it is first of all allyl carbocation and in resonance with the benzene and the double bond.
(3) It is allyl carbocation, so it is less favorable to SN1 than (1) and more favorable to SN2.
(4) It is the most favorable towards SN2 reaction.
Hence the order of reactivity towards SN2 reaction should be
(4) > (3) > (1) > (2)
But the answer is
(2) > (1) > (3) > (4)