To determine which substance is the acid in the reaction, we should first understand the characteristics of acids:
Acids are substances that release hydrogen ions (H⁺) when dissolved in water. For this purpose, we will examine each substance in the reaction to identify the acid:
1. [tex]\( Mg(s) \)[/tex] - Magnesium in its solid form is a metal and does not release hydrogen ions in solution.
2. [tex]\( H_2(g) \)[/tex] - Hydrogen gas is not an acid because it is in its molecular form and does not release hydrogen ions.
3. [tex]\( MgSO_4(aq) \)[/tex] - Magnesium sulfate in aqueous solution is a salt and does not release hydrogen ions.
4. [tex]\( H_2SO_4(aq) \)[/tex] - Sulfuric acid in its aqueous form is a well-known acid that dissociates in water to release hydrogen ions (H⁺).
Since only [tex]\( H_2SO_4(aq) \)[/tex] releases hydrogen ions when dissolved in water, it is the substance that fits the definition of an acid in this reaction.
So, the acid in the reaction is:
[tex]\[ H_2SO_4(aq) \][/tex]
Therefore, the correct choice is the fourth one:
[tex]\[ \boxed{H_2SO_4(aq)} \][/tex]
