Answer: The correct answer is D.

Explanation: In a chemical reaction, when there is changing of ions taking place and is leading to the formation of a gas can be considered as a single displacement reaction.

Single-Displacement reactions are defined as the reactions in which a higher reactive metal displaces a lower reactive metal. This is in reference to the reactivity series.

Hydrogen lies below in the reactivity series and when the higher reactive metal displaces hydrogen from a chemical reaction, it leads to the formation of hydrogen gas.

Example:

Zn^{2+}+2HClrightarrow ZnCl_2+H_2(g)