To identify the acid in the given reaction, we need to understand the basic definitions of acids. According to the Bronsted-Lowry theory, an acid is a substance that can donate a proton (H[tex]\(^+\)[/tex]).
The reaction given is:
[tex]\[
H_2SO_4(aq) + Mg(s) \longrightarrow MgSO_4(aq) + H_2(g)
\][/tex]
Now, let's analyze each substance involved in this reaction:
1. [tex]\[ Mg(s) \][/tex]: Magnesium in its solid form is a metal and not an acid.
2. [tex]\[ H_2(g) \][/tex]: Hydrogen gas is a simple diatomic molecule and does not act as an acid.
3. [tex]\[ MgSO_4(aq) \][/tex]: Magnesium sulfate in aqueous solution is a salt and does not have acidic properties.
4. [tex]\[ H_2SO_4(aq) \][/tex]: Sulfuric acid in aqueous solution is a known strong acid. It can donate protons (H[tex]\(^+\)[/tex]) to other substances in a reaction.
Considering these points, the substance that acts as an acid in the reaction is:
[tex]\[ H_2SO_4(aq) \][/tex]
Therefore, the substance which is an acid in this reaction is [tex]\( H_2SO_4(aq) \)[/tex], which is the fourth option.
Hence, the correct answer is:
[tex]\[ \boxed{4} \][/tex]
