When naming a transition metal ion that can have more than one common ionic charge, the numerical value of the charge is indicated by a
a. superscript after the name.
b. suffix.
c. prefix.
d. roman numeral following the name.

The answer is: d. Roman numeral following the name.

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).

Another example, name of the compound Re₂O₅ is rhenium(V) oxide.

In this chemical compound atom of rhenium has oxidation number +5 and atom oxygen has oxidation number -2, chemical compound rhenium(V) oxide has neutral charge (2·(+5) + 5·(-2) = 0).

Roman five is for valence of rhenium in this compound.  

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