Final answer:
The reaction where C2H2 reacts with H2 to form C2H4 is known as an addition reaction, which adds atoms to an unsaturated molecule to form a more saturated molecule.
Step-by-step explanation:
When C₂H₂ reacts with H₂, the compound C₂H₄ is produced. This reaction is correctly classified as an addition reaction. In an addition reaction, atoms or groups of atoms are added to unsaturated molecules, such as alkenes or alkynes, to form a more saturated molecule. This is in contrast to combustion reactions, which involve the combination of a hydrocarbon with oxygen to produce carbon dioxide and water accompanied by a release of energy.