Correct Answer - B
Order of reactivity in `S_(N^(2))` reactions w.r.t. RX is
`1^(@)gt2^(@)gt3^(@)`
In these reactions bulky R-groups hinders the attack of nucleophile. The presence of Ph- CO-group in S stabilises the intermediate transition state and thus S is most reactive. Thus correc order is :
S gt P gt R Q