The reagent used in Clemmensen’s reduction is: (A) Conc. H₂SO₄ (B) Zn - Hg/conc.HCl (C) aq. KOH (D) alc. KOH

The Clemmensen reduction is a chemical reaction used to reduce ketones or aldehydes to alkanes. This reduction takes place under acidic conditions. The reagent used in this reaction is a combination of zinc amalgam (Zn-Hg) and concentrated hydrochloric acid (HCl).

The reaction can be summarized as follows:
\[
\text{R-CO-R'} + \text{Zn-Hg/conc. HCl} \rightarrow \text{R-CH2-R'}
\]
Here, the carbonyl group (C=O) is reduced to a methylene group (CH₂).

**Answer:**
The correct reagent is (B) \[Zn - Hg/conc.HCl\].

Explanation:
- \[Zn-Hg\] acts as a reducing agent.
- \[Conc. HCl\] provides the acidic medium necessary for the reaction to proceed.

Other options:
- (A) \[{H_2}S{O_4}\] is a strong acid, but not used in Clemmensen reduction.
- (C) \[aq.KOH\] is used in base-mediated reductions or eliminations, not for Clemmensen reduction.
- (D) \[alc.KOH\] is used for dehydrohalogenation reactions, not for the Clemmensen reduction.

Thus, option (B) is the correct answer.