Which of the following reactions is the metal displacement reaction ? Choose the right option.

The correct reaction is:

Understanding Displacement Reactions:
A metal displacement reaction is a type of redox reaction in which a more reactive metal displaces a less reactive metal from its compound (such as its oxide, chloride, or salt solution).

Analysis of the Options:

1. Reaction 1:

In this reaction, iron ($''Fe''$) reacts with hydrochloric acid ($''HCl''$) to displace hydrogen gas ($''H''$₂). Since hydrogen is a non-metal, this reaction represents a hydrogen displacement reaction (non-metal displacement), not a metal displacement reaction.

2. Reaction 2:

This is a decomposition reaction (specifically thermal decomposition) where lead nitrate breakdowns into lead oxide, nitrogen dioxide, and oxygen gas.

3. Reaction 3:

This is also a thermal decomposition reaction where potassium chlorate breaks down into potassium chloride and oxygen.

4. Reaction 4:

In this reaction, aluminum ($''Al''$), which is a more electropositive and reactive metal, reduces chromic oxide ($''Cr''$₂O₃) upon heating to displace chromium ($''Cr''$) metal:
$''Cr''$₂O₃ + 2Al → Al₂O₃ + 2Cr
Since a metal metal ($''Al''$) displaces another metal ($''Cr''$) from its oxide compound, this is a classic metal displacement reaction.

Therefore, the correct option showing a metal displacement reaction is: