The correct option is A P<Q<R<S
Heat of hydrogenation∝1stability of alkene
The stability of alkenes depends upon two major factors:
ii) Number of α−hygrogens
(P) being most stable has least heat of hydrogenation since the double bond is in conjugation.
Among (Q), (R) and (S), the number of α−hygrogens are in the order: Q>R>S
Hence, the heat of hydrogenation follows the trend: P<Q<R<S