They don't change what the substance really is unlike chemical change. They chemical formula of the substance stays the same even though the substance can go under shape change.
Answer:
c. Cr
Explanation:

The compound is containing
ion.
If acid reacts with it, CO_2 evolves.

therefore, Cr would form a precipitate when added to an aqueous solution of this compound. Cr2CO3 is the precipitate.