Answer:
C. The positive charge on an ion
Step-by-step explanation:
Roman numerals are used often in compounds involving transition metals for example copper (II) sulphate, copper (I) oxide, iron (III) Chloride. This is because such transition metals are capable of forming different types (in terms of charged) of cations. Iron for example can form the a 2+ cation or a 3+. So it is important to specify which one is in that compound because that affects the chemistry of the compound.