- II>IV>III>I
Explanation:
The correct reactivity order of alcohols towards H−X will be II>IV>III>I
Alcohol II has maximum reactivity as the carbocation formed will be stabilized by resonance with adjacent C=C double Bond.
Alcohol I has minimum reactivity as the carbocation formed will have positive charge on sp2 carbon atom.
Alcohol IV is more reactive than alcohol III because alcohol IV gives secondary carbocation whereas alcohol III gives primary carbocation.
Secondary carbocation is more stable than primary carbocation.