Concept: Terminal alkynes react with copper(I) chloride solution to form a precipitate of copper acetylide.Explanation:• But-1-yne has a terminal alkyne group: CH≡C−CH2−CH3.• Terminal alkynes have an acidic hydrogen on the C≡C carbon, which reacts with copper(I) chloride to give a precipitate.• But-2-yne is an internal alkyne and does not give this precipitate.• But-1-ene and But-2-ene are alkenes, which do not give this precipitate.Answer: D. But-1-yne