Roman numerals are used to indicate oxidation states. Hence, option C is correct.
An oxidation state is a number given to the atom of an element when participating in a chemical reaction.
Roman numerals are used to indicate oxidation states of transition metals.
The names for transition metal compounds often have roman numerals in them, because the roman numerals indicate the charge on the transition metal. But the oxidation state of Cu in CuO is +2, and the oxidation state of Cu in [tex]Cu_2O[/tex] is +1. Name metal first and charge and transition metal with a roman numeral.
The transition metals may form more than one ion, thus it is needed to be specified which particular ion we are talking about. This is indicated by assigning a Roman numeral after the metal. The Roman numeral denotes the charge and the oxidation state of the transition metal ion.
[tex]Fe^{2+}[/tex] would be named iron (II) and [tex]Fe^{3+}[/tex] would be named iron (III).
Hence, option C is correct.
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