Concept:Metals react with acids to liberate hydrogen gas.
Explanation:When magnesium metal is added to an acidic solution, a chemical reaction takes place.
The acid donates hydrogen ions, which are reduced to hydrogen gas.
For example, hydrochloric acid reacts with magnesium as follows:
Mg+2HCl→MgCl2+H2↑The gas produced is hydrogen, which burns with a pop sound.
Among the given options, only hydrochloric acid is an acid.
Sodium chloride solution, sodium hydroxide solution, and calcium nitrate solution are not acids.
Therefore, the unknown solution must be hydrochloric acid.
Answer:C. Hydrochloric acid