Final answer:
HClO4 contains the perchlorate ion, which has the most oxygen atoms in the series of chlorine's oxoanions and is named using the prefix 'per-' indicating its high oxygen content.
Step-by-step explanation:
The polyatomic ion involved in HClO4 is perchlorate, and its name reflects this composition. This ion is part of a series of oxoanions of chlorine, with perchlorate being the variant with the most oxygen atoms. The prefix "per-" in perchloric acid indicates that the oxoanion contains the highest number of oxygen atoms in the series, distinguishing it from chlorate, chlorite, and hypochlorite, which have progressively fewer oxygen atoms.