${\mathrm{S}}_{\mathrm{N}}1$ reaction rate depends upon the stability of the carbocation, as carbocation formation is the rate determining step compound (ii), forms at $2°$ allylic carbocation which is the most stable, the next stable carbocation is formed from (iii), it is a $2°$ carbocation, (i) forms the least stable $1°$ carbocation, the order of reactivity is thus,