The composition of Lucas reagent is

$1\, HCl: 1\, ZnCl_2$

The correct answer is Option (4) → $1\, HCl: 1\, ZnCl_2$

Lucas reagent is prepared by mixing concentrated hydrochloric acid and anhydrous zinc chloride in equal proportions.

Reasoning:
Lucas reagent is used to distinguish between primary, secondary and tertiary alcohols. Zinc chloride acts as a catalyst and concentrated hydrochloric acid provides the chloride ions needed for substitution. An equal proportion ensures effective reaction speed and proper turbidity observation.

Option-wise Detailed Explanation

1. 2 HCl : 1 ZnCl₂ This ratio implies an excess of Hydrochloric acid. While the reaction would still technically happen, the reagent would be "diluted" by the extra acid. The $ZnCl_2$ is the catalyst that breaks the $C-O$ bond. If its concentration is too low relative to the $HCl$, the test for secondary alcohols becomes very slow and unreliable, making it harder to distinguish them from primary alcohols.

2. 1 HCl : 2 ZnCl₂ This ratio suggests a massive excess of Zinc Chloride. $ZnCl_2$ is a salt. If you try to dissolve 2 moles of it into only 1 mole of $HCl$, you will likely reach a point of supersaturation where the salt won't even dissolve properly. The reagent needs to be a clear solution to start with so that you can actually see the "cloudiness" form later.

3. 3 HCl : 1 ZnCl₂ This is even more diluted than Option 1. With this much $HCl$, the catalytic power of the $ZnCl_2$ is significantly weakened. The Lucas test relies on the Lewis acidity of $ZnCl_2$; adding too much $HCl$ shifts the balance and makes the reagent act more like plain $HCl$, which doesn't react with primary or secondary alcohols at room temperature

4. 1 HCl : 1 ZnCl₂ This is the equimolar composition. It provides the maximum possible concentration of both the catalyst ($ZnCl_2$) and the reactant ($HCl$) in a stable solution. This specific balance is what allows the reagent to stabilize the transition state of the alcohol, leading to the distinct 0, 5, and 30-minute reaction windows that identify the alcohol type.