What kind of reaction occurs when alkenes react with hydrogen, water, and halogens?

The correct choice is addition because alkenes have at least one carbon-carbon double bond, which is a point of reactivity. When alkenes react with hydrogen, water, or halogens, they undergo a chemical reaction where these molecules are added across the double bond, converting it into a single bond. This process results in the formation of saturated compounds.

For instance, when an alkene reacts with hydrogen (a process known as hydrogenation), the double bond is broken, and hydrogen atoms are added to the carbon atoms that were part of the double bond. Similarly, during hydration (reaction with water), elements from the water molecule are added, resulting in an alcohol. When reacting with halogens, such as chlorine or bromine, the halogen atoms add to the alkene, breaking the double bond and forming dihalogenated products.

In each of these reactions, the key feature is the addition of new atoms or groups to the original molecule, which distinguishes addition reactions from other types of reactions, such as substitution or decomposition, where the core structure of the original molecule remains more intact or is broken down to yield different products.