Which of the following gas is evolved when an acid reacts with a metal?

Hydrogen

The correct answer is option 3. Hydrogen.

When an acid reacts with a metal, a chemical reaction occurs in which hydrogen gas is evolved. This type of reaction is known as a metal-acid reaction or a single displacement reaction. The general equation for this reaction can be represented as:

\[ \text{Metal} + \text{Acid} \rightarrow \text{Salt} + \text{Hydrogen Gas} \]

For example, consider the reaction between hydrochloric acid (\( HCl \)) and zinc metal (\( Zn \)):

\[ Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g) \]

In this reaction, zinc (Zn) reacts with hydrochloric acid (\( HCl \)) to produce zinc chloride (\( ZnCl_2 \)) and hydrogen gas (\( H_2 \)).

The evolution of hydrogen gas is one of the characteristic features of such reactions. This gas is typically observed as bubbles or effervescence during the reaction. The hydrogen gas is formed as a result of the displacement of hydrogen ions (\( H^+ \)) from the acid by the metal atoms. The displaced hydrogen ions combine with each other to form hydrogen gas molecules (\( H_2 \)).

Overall, the evolution of hydrogen gas is a key indicator of the occurrence of a metal-acid reaction.