Which of the following compounds contains the ammonium radical?

The correct answer indicates that quaternary compounds contain the ammonium radical. Ammonium is a positively charged polyatomic ion with the formula NH4+. In the context of quaternary compounds, they can be understood as quaternary ammonium compounds, which have a nitrogen atom bearing a positive charge and are bonded to four organic groups. This structure results from the substitution of hydrogen atoms in ammonium by organic groups, making them functional derivatives of ammonium.

The other options do not contain the ammonium radical. Hydrochloric acid is a simple binary acid composed of hydrogen and chlorine, with no ammonium ion present. Sulfates are a class of salts containing the sulfate ion (SO4^2-), and carbonates consist of the carbonate ion (CO3^2-). Neither of these ions involves ammonium in their structure. Thus, quaternaries are unique in this context for containing the ammonium radical, confirming their identity as quaternary ammonium compounds.