which molecule is an aromatic hydrocarbon? mark this and return

Explanation:

Step1: Recall aromatic hydrocarbon criteria

Aromatic hydrocarbons have a cyclic, planar structure with a conjugated $\pi -$electron system following Hückel's rule ($4n + 2$ $\pi$ - electrons, where $n=0,1,2,\cdots$).

Step2: Analyze first structure

The first structure is an alkene, not cyclic, so it is not an aromatic hydrocarbon.

Step3: Analyze second structure

The second structure is an alkyne - containing chain - compound, not cyclic, so it is not an aromatic hydrocarbon.

Step4: Analyze third structure

The third structure is a cyclic compound with a conjugated $\pi -$electron system. It has 10 $\pi$ - electrons ($n = 2$ in $4n+2$), is cyclic, planar and conjugated, so it is an aromatic hydrocarbon.

Answer:

The third molecule (the cyclic one with conjugated double - bonds) is an aromatic hydrocarbon.