The decreasing order of stability of following cations is :
(a) S > R > Q > P   (b) P > Q > R > S   (c) Q >R > S > P   (d) R > S > P > Q

Here all carbocations are  primary benzylic carbocations  in which positive charge  gets delocalized in the ring.During delocalization positive charge goes on para carbon of the ring.Now this resonating structure is more stable with more hyperconjugated Hs on the carbon bonded to para carbon of the ring.

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