The decreasing order of stability of following cations is (Question No. 56-60).
(a) Q>R >P >S                                                     
(b) S>Q>R>P
(c) S >R >P.>Q
(d) R > P > Q > S

Nitro group is electron withdrawing group due to positive charge on N atom.
EWG destabilizes the carbocation and destabilization is more if during resonance positive charge is on the carbon bonded to nitro group . N has  -I effect also which is strongest on the ortho position due to proximity with carbocation carbon.
If all factors are considered then (S) is most stable and (P) is least stable.Can you now figure out the right answer?

Its  a great explanation so (B) is correct answer thanks.