The correct answer is $\boxed{\text{C}}$. Butane is a saturated hydrocarbon, while the other three options are unsaturated hydrocarbons.
A saturated hydrocarbon is a hydrocarbon that contains the maximum number of hydrogen atoms possible for its carbon skeleton. Unsaturated hydrocarbons contain fewer hydrogen atoms than the maximum possible, and have double or triple bonds between carbon atoms.
Acetylene is a hydrocarbon with the chemical formula $C_2H_2$. It is a colorless, flammable gas with a garlic-like odor. Acetylene is the simplest alkyne, a type of unsaturated hydrocarbon that contains a triple bond between two carbon atoms.
Pentene is a hydrocarbon with the chemical formula $C_5H_10$. It is an alkene, a type of unsaturated hydrocarbon that contains one or more double bonds between carbon atoms. There are three structural isomers of pentane: n-pentane, isopentane, and neopentane.
Benzene is a hydrocarbon with the chemical formula $C_6H_6$. It is an aromatic hydrocarbon, a type of unsaturated hydrocarbon that contains a six-membered ring of carbon atoms with alternating single and double bonds. Benzene is a colorless, flammable liquid with a sweet odor. It is a major component of gasoline and is used in the production of many other chemicals.