Catenation is the ability of a chemical element to form a long, chain-like structure via a series of covalent bonds. Catenation occurs readily with the element carbon and it's various compounds, which form covalent bonds with other carbon atoms. Catenation is the reason for the existence of such a large number of organic compounds in nature. Carbon is well known for it's properties of catenation, with organic chemistry essentially being the study of catenated carbon structures, otherwise known as catenae.
So, reviewing what we have to pick from in your question, I would say the CH3CH2CH3 most logically represents the stringing together of multiple carbon atoms, linked by covalent bonding. In all the other examples, only one carbon atom is listed, making them improbable picks for the correct answer.