The correct answer is D. Propine.
Propane has a molecular formula of $C_3H_8$. It is a saturated hydrocarbon, meaning that it has all single bonds. Propylene has a molecular formula of $C_3H_6$. It is an unsaturated hydrocarbon, meaning that it has at least one double bond. Propanol has a molecular formula of $C_3H_7OH$. It is an alcohol, meaning that it has a hydroxyl group (-OH) attached to a carbon atom. Propanoic acid has a molecular formula of $C_3H_6O_2$. It is a carboxylic acid, meaning that it has a carboxyl group (-COOH) attached to a carbon atom.
A triple bond is a covalent bond between two atoms that involves the sharing of six electrons. Triple bonds are shorter and stronger than double bonds, which are in turn shorter and stronger than single bonds.
Propine is the only compound in the list that has a triple bond. It has a molecular formula of $C_3H_4$ and is an unsaturated hydrocarbon. It is a colorless gas that is highly flammable. It is used as a fuel and as a starting material in the production of other chemicals.