The molecule with 6 carbon atoms is C. Hexene. Hexene is a hydrocarbon with six carbon atoms in its structure. The prefix "hex-" in chemistry indicates that the molecule contains six carbon atoms. In this case, "hexene" specifically refers to a compound with a double bond in the carbon chain. This molecule is distinct from the other options listed (Pentene, Heptane, and Octyne) which have different numbers of carbon atoms in their structures. Therefore, in this context, hexene is the correct answer as it corresponds to a molecule with 6 carbon atoms.
