When is a roman numeral most likely needed in the name of an ionic compound? a)when more than one polyatomic ion appears
b)when the anion is a polyatomic ion
c)when more than one cation is used in the formula
d)when the cation is a transition metal

Answer is: D) when the cation is a transition metal.

The Roman numeral shows the charge and the oxidation state of the transition metal ion.

For example, cobalt can form two chlorides, with charge of 2+ (cobalt(II) chloride - CoCl₂) and with charge 3+ (cobalt(III) chloride - CoCl₃).

Transition metals are elements in the d-block of the periodic table.

D) When the cation is a transition metal.

For example, FeCl2 - iron (II) chloride vs FeCl3 - iron(III) chloride.

i think it's letter a.