Unsaturated hydrocarbons react very rapidly with halogens. The unsaturated compound can be changed to a saturated compound. No energy is needed for this reaction, as the double or triple bonds are very unstable.
I would pick "<span>Unsaturated hydrocarbons are very reactive." This is because while unsaturated hydrocarbons are indeed more reactive than their saturated counterparts, they still do require some energy, called activation energy, to be overcome before the reactions occur.</span>
