Which compound is named with a Roman numeral in its name?

The compound that is named with a Roman numeral in its name is indeed a transition metal compound, which often requires the use of Roman numerals to indicate the oxidation state of the metal. In this case, CuCl is copper(I) chloride, where copper can have multiple oxidation states, namely +1 and +2. The Roman numeral I in the name indicates that copper is in the +1 oxidation state in this particular compound.

The other compounds listed do not require a Roman numeral in their names. Sodium (Na) and potassium (K), the metals in the other choices, have only one common oxidation state (+1 for sodium and +1 for potassium), so they do not need a Roman numeral for their nomenclature. As a result, NaH (sodium hydride), K2O (potassium oxide), and NaBr (sodium bromide) are all named without Roman numerals. This highlights the specific role of Roman numerals in naming certain metal compounds based on their variable oxidation states, particularly in transition metals like copper.