The order of reactivity of dehydration of alcohol is

The order of reactivity is 1° < 2° < 3°.

The creation of carbocations during dehydration is the slowest process, therefore the more stable the carbocation, the more quickly the reaction will occur. Carbocation stability, from highest to lowest:  3° > 2° > 1°.correct option is D.

Step 1: Formation of protonated alcohol 

Step 2: Formation of carbocation. It is slowest step and rate determining step.

Step 3: Formation of ethene by elimination of proton