There are several possibilities for X. I'll nominate copper(II) chloride.
Copper(II) chloride is
Copper(II) carbonate is a green, insoluble substance so a green precipitate would result if carbonate ions were added:
Another candidate is magnesium nitrate.
The idea here is that you're looking for a cation that will not form a precipitate when mixed with those three anions, chloride, sulfate, and nitrate, but that it will form one when mixed with a solution that contains carbonate anions.
My cation of choice is the magnesium cation,
#"Mg"_text((aq])^(2+) + "CO"_text(3(aq])^(2-) -> "MgCO"_text(3(s]) darr#
If you assume that solution