Which metal atom below cannot form a cation of several different charges?

Missing question: Cr, Mn, Fe, Ba, Cu.

Answer is: Ba (barium).

Barium is metal from second group of periodic table of elements. Barium looses two electrons and has oxidation number + 2.

Chromium has different oxidation numbers, usually + 3 and + 6, iron has oxidation numbers + 2 and + 3, copper + 1 and + 2 in their compounds.