The rate of acid mediated dehydration reaction of alcohol depends on the stability of the carbocation. The order of stability of carbocations for given alcohols is
CH3−
CH3
|
C⊕
|
CH3
III
>CH3−
CH2−
⊕
C
H
(IV)
−CH3
>CH3−
CH3
|
C
H
(II)
−CH2⊕>CH3−
CH2−CH2
(I)
−
⊕
C
H2
Hence, the correct order of rate of dehydration is III > IV > II > I.