Explanation:
In order to produce CuCl2 crystals, there have to be copper and chlorine atoms.
Copper oxide (CuO) has copper atoms. We need the acid to have chlorine atoms.
Out of HCl, HNO3 and H2SO4, only HCl has chlorine atoms.
CuO + 2HCl => CuCl2 + H2O.
Hence the answer is Hydrochloric acid.