Which one of the following ions exhibits resonance?

Question

OH-
(SO4)2-
(O)2-
NH4+
(CO3)2-

Answer 1

(CO3)2-

The structure is O=C(O-)2 with the O=C alternating among all three oxygen atoms in the three different resonance structures.

Some may argue that (SO4)2- is composed of resonance structures based on two S=O bonds and two S-O - bonds, with the S=O bonds working their way around the ion in four resonance structures.

The argument against (SO4)2- existing as resonance structures might be that there are not two S=O bonds, but two S-->O bonds, where S-->O is a coordinate covalent bond (both electrons to be shared by S and O contributed by S).

Are you sure you're allowed only one choice?