The decreasing order of reactivity towards dehydrohalogenation $\left(E_1\right)$ reaction of the following compounds is:

The correct option is D $D>B>C>A$
In $E_1$ mechanism, the rate determining step is formation of carbocation. So, stability of carbocation formed decides the rate.
In option D, the cation formed is resonance stabilised.
In option C, the cation formed is a $2^{\circ }$ carbocation.
In option A and B, the carbocations formed are $1^{\circ }$ but there is a chance of rearrangement in option b and after the rearrangement, the carbocation formed in option b will be allylic. So, the order of reaction is as follows: $D>B>C>A$