The best reagent for preparing an alkyl chloride from an alcohol is

$SOCl_2$

The correct answer is Option (2) → $SOCl_2$

The best reagent for preparing an alkyl chloride from an alcohol is $SOCl_2$ (Thionyl Chloride).

While all the listed reagents can convert an alcohol ($R-OH$) into an alkyl chloride ($R-Cl$), thionyl chloride is industrially and laboratory-preferred for one primary reason: the purity of the product.

When an alcohol reacts with $SOCl_2$:

$R-OH + SOCl_2 \rightarrow R-Cl + SO_2(g) + HCl(g)$

• Gaseous By-products: Both sulfur dioxide ($SO_2$) and hydrogen chloride ($HCl$) are gases.
• Self-Purification: These gases escape the reaction mixture immediately, leaving behind the alkyl chloride in a relatively pure state. This eliminates the need for complex separation or purification steps.
• High Yield: According to Le Chatelier’s principle, the continuous escape of gaseous products shifts the equilibrium to the right, ensuring a very high yield of the alkyl chloride.