What is the primary characteristic of an aromatic hydrocarbon?

An aromatic hydrocarbon is primarily characterized by the presence of a conjugated pi electron system, which gives rise to its unique stability and reactivity. This system consists of alternating single and double bonds within a cyclic structure, allowing for delocalization of electrons across the molecule. This delocalization is a key feature that contributes to the distinct properties of aromatic compounds, such as their relatively low reactivity compared to alkenes or alkynes, and their characteristic fragrance.

In contrast, the other choices do not accurately reflect the essential nature of aromatic hydrocarbons. Aromatic compounds are not defined by just having single carbon bonds; they typically feature alternating bonds. Additionally, they are not necessarily saturated with hydrogen, as many aromatic hydrocarbons can have hydrogen atoms replaced by other substituents. Lastly, they do not require a branched structure; aromatic compounds can be either branched or straight-chain, but what defines them is the aromatic character bestowed by the conjugated pi system.