Alkenes: which statement is true about their reactivity?

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Alkenes are unsaturated compounds because they have a carbon–carbon double bond, which contains a pi bond. That pi bond is relatively reactive and readily interacts with reagents, allowing the bonds to break and two new sigma bonds to form. The most characteristic and broadly applicable type of reaction for alkenes is addition across the double bond, where reagents add to each carbon of the C=C to give more saturated products. This explains why hydrogenation, halogenation, hydrohalogenation, and hydration are all typical pathways for alkenes.

Polymerization is also possible for many alkenes, where many small alkene units add together to form long polymer chains. This is a specific pattern of addition chemistry and explains one important, practical outcome of their reactivity, but it depends on conditions and the particular alkene. The statement that alkenes are fully saturated is incorrect, since the double bond means they are unsaturated. They do not only undergo substitution reactions; the hallmark behavior is addition across the double bond.