Recall the definition of Hinsberg's reagent

Hinsberg's reagent is the reagent used in the Hinsberg test to distinguish between primary, secondary, and tertiary amines.

Chemical identity of Hinsberg's reagent

Hinsberg's reagent is benzenesulphonyl chloride, whose formula is: $C_6H_5SO_2Cl$

Check the options

• A: $C_6H_5COCl$ = benzoyl chloride, not Hinsberg's reagent
• B: $C_6H_5SO_2Cl$ = benzenesulphonyl chloride, correct
• C: $SOCl_2$ = thionyl chloride, not Hinsberg's reagent
• D: $(COCl)_2$ = oxalyl chloride, not Hinsberg's reagent

Conclusion

Therefore, Hinsberg's reagent is: $\boxed{C_6H_5SO_2Cl}$

So the correct option is B.