Amongst the following alcohols which would react fastest with conc. HCl and ZnCl$$_2$$?

Lucas reagent is a mixture of concentrated $$HCl$$ and anhydrous $$ZnCl_2$$. It converts an alcohol $$R{-}OH$$ into the corresponding alkyl chloride $$R{-}Cl$$.

First, state the general reaction that takes place in the presence of Lucas reagent:

$$R{-}OH + HCl \;\xrightarrow[\text{conc.}]{ZnCl_2}\; R{-}Cl + H_2O$$

The mechanism proceeds by the $$S_N1$$ pathway for secondary and tertiary alcohols and by a much slower $$S_N2$$ pathway for primary alcohols. In an $$S_N1$$ reaction the rate-determining step is the formation of a carbocation, and the stability of this carbocation directly controls the speed of the reaction. The order of carbocation stability is

$$\text{tertiary} \;>\; \text{secondary} \;>\; \text{primary}$$

Therefore, the order of reactivity of alcohols with Lucas reagent follows the same trend:

$$\text{tertiary alcohol} \;>\; \text{secondary alcohol} \;>\; \text{primary alcohol}$$

Now classify each given alcohol:

1. Pentanol is $$\text{CH}_3\!-\!\text{CH}_2\!-\!\text{CH}_2\!-\!\text{CH}_2\!-\!\text{CH}_2OH$$.   The hydroxyl group is attached to an end carbon, so it is a primary alcohol.

2. 2-Methyl butanol is $$\text{(CH}_3)_2\text{CHCH}_2CH_2OH$$ (also written as 2-methyl-1-butanol).   Again the OH is on a terminal carbon, so this is a primary alcohol.

3. 2-Pentanol is $$\text{CH}_3\!-\!\text{CH}_2\!-\!\text{CH}(OH)\!-\!\text{CH}_2\!-\!\text{CH}_3$$.   The OH is on the second carbon, which is bonded to two other carbons, hence it is a secondary alcohol.

4. 2-Methyl butan-2-ol is $$\text{(CH}_3)_3\text{C}OH$$.   The carbon bearing the OH is attached to three other carbons, so it is a tertiary alcohol.

Using the previously stated reactivity order, the tertiary alcohol will react the fastest with conc. $$HCl$$ and $$ZnCl_2$$. Among the four options, only 2-methyl butan-2-ol is tertiary.

Hence, the correct answer is Option D.