In this benzene derivative, an ethyl group (CH2-CH3) is attached to the benzene ring (C6H5-). According to IUPAC nomenclature, when a simple alkyl group like methyl (CH3-), ethyl (CH3-CH2-), propyl (CH3-CH2-CH2-), etc. is attached to a benzene ring, the name of the alkyl group is prefixed to the parent name "benzene." Therefore, the systematic name for this compound is ethylbenzene.