The correct answer is Option B: C₆H₅SO₂Cl.

Hinsberg's reagent is benzenesulfonyl chloride (C₆H₅SO₂Cl). It is used to differentiate primary, secondary, and tertiary amines.

Here's why the other options are incorrect :

• Option A: C₆H₅COCl is benzoyl chloride, a reagent used in Friedel-Crafts acylation reactions.


• Option C: SOCl₂ is thionyl chloride, a reagent used to convert alcohols into alkyl chlorides.


• Option D: (COCl)₂ is oxalyl chloride, a reagent used for the conversion of carboxylic acids into acid chlorides.