Which of the following is classified as an alkyne?

B. Ethyne is classified as an alkyne because it contains a carbon-carbon triple bond in its molecular structure. Alkynes are hydrocarbons characterized by at least one triple bond between two carbon atoms. Ethyne, also known as acetylene, is the simplest alkyne, with the formula C2H2, demonstrating this distinct bonding structure.

The other choices represent different classes of hydrocarbons. Propylene, for instance, is an alkene with a carbon-carbon double bond. Butene is also an alkene, characterized by its double bond. Ethylene, similarly, is another alkene and has a double bond as well. This distinction in bonding is fundamental in classifying these compounds into their respective categories: alkynes, alkenes, or aliphatic hydrocarbons.