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

The compound that is named with a Roman numeral in its name is copper(I) chloride, which corresponds to option A (CuCl). The use of Roman numerals in naming indicates the oxidation state of an element that has multiple possible charges. In this case, copper can exist in several oxidation states, including +1 and +2. When forming compounds, it is important to specify which oxidation state is present to avoid ambiguity.

In copper(I) chloride, the Roman numeral (I) indicates that copper is in the +1 oxidation state while it is combined with chloride ions, which have a -1 charge. Thus, one copper ion combines with one chloride ion to create a neutral compound.

In contrast, the other options—sodium hydride (NaH), potassium oxide (K2O), and sodium bromide (NaBr)—do not require Roman numerals for their naming because the metals involved have a fixed oxidation state. For example, sodium always has a +1 charge, and potassium always has a +1 charge as well. Hence, there is no need to specify their oxidation states with Roman numerals in these cases.