The order of reactivity of following alcohols with halogen acids is:

I. CH3CH2CH2OH
II.
III.

1.  (I) >(II)> (III) 

2.  (III) >(II) >(I) 

3.  (II) >(I)> (III)

4.  (I) >(III)> (II) 

Hint: Rate of reaction depends on the stability of carbocation

Halogenation of alcohol by haloacids is done by the SN1 mechanism. This SN1 mechanism involves carbocation intermediate formation.

The generation of carbocations with the present three alcohol substrates in terms of the reaction mechanism is as follows:.





Here 1° carbocation formation occurs through protonation of 1° -OH followed by a nucleophilic attack.

Here  2° carbocation formation occurs which is comparatively more stable than primary alcohol.

the rate of SN1 mechanism will increase as stability of carbocation will increase

Similarly, the tertiary carbocation formed from tertiary alcohol is most stable so therefore here the rate of reaction by SN1 mechanism is highest.

So, the correct option is second


Conclusion:- Higher the stability of carbocation intermediate higher will be the rate of halogenation of given alcoholic substrates by haloacids.