It is an example of SN1 reaction.
Rate of SN1reactionαstabilityofcarbocation
In option 1 carbocation formed after removing halogen is destabilized by −I effect.
In option 2 carbocation formed after removing halogen is stabilized by +M effect since lone pair is present at oxygen atom.
In option 3 carbocation formed after removing halogen is stabilized by +M effect of lone pair present on nitrogen atom but lone pair is also in conjugation with benzene ring therefore +M effect is less.
In option 4 carbocation formed after removing halogen is stabilized by +M effect of lone pair present on nitrogen atom and +M effect of nitrogen atom is more than that of oxygen atom. Hence, option 4 is most stable.



