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

Here all are benzylic carbocation..
Now remember EDGs increases the stability of the carbocation and EWGs decreases the stability of carbocations.
-OH the strongest EDG and NO2 is the strongest EWG. -NHCOR (amides) are milder EDG.
Now arrange them in right order ...
P> R> Q> S 

Really helpful Thank you very much.