The correct answer is [C]. The reaction that converts alkenes into alkanes is called hydrogenation. In this process, hydrogen gas (H₂) is added to the carbon–carbon double bond of the alkene, converting it into a single bond, thereby forming an alkane. For hydrogenation to occur, specific conditions are required: a metallic catalyst, usually nickel (Ni), must be present to speed up the reaction, and the reaction temperature should be maintained around 423 K. This process is widely used in the food and chemical industries.