In Clemmensen reduction the carbonyl compound is treated with

Zinc amalgam + hydrochloric acid

The correct answer is Option (3) → Zinc amalgam + hydrochloric acid

In Clemmensen reduction, the carbonyl group of aldehydes or ketones is reduced to a $CH_2$ (methylene) group. The specific reagents used for this transformation are Zinc amalgam ($Zn(Hg)$) and concentrated hydrochloric acid ($HCl$).

How the Reaction Works

The Clemmensen reduction is particularly useful for reducing carbonyl compounds that are stable toward strong acids but might be sensitive to basic conditions (where the alternative, Wolff-Kishner reduction, would be used).

• Reagents: $Zn(Hg) / \text{conc. } HCl$
• Transformation: $>C=O \rightarrow{} >CH_2$
• By-product: Water ($H_2O$)

Example

If you treat Propanal ($CH_3CH_2CHO$) with zinc amalgam and hydrochloric acid, it is reduced to Propane ($CH_3CH_2CH_3$).

Why the other options are incorrect:

• Nitric Acid: This is a strong oxidizing agent. Using it would likely oxidize the organic molecule or react violently with the zinc rather than facilitating a reduction.
• Sodium Amalgam: While $Na(Hg)$ is a reducing agent, it is typically used for different types of reductions (like the reduction of esters or in the presence of water/alcohols) and is not the reagent defined for the Clemmensen process.