The correct option is D Silver vessel
The goal is to keep the copper ions in solution. So the metal should not displace copper ions. In terms of reduction potentials, the more positive the potential is, the higher that metal's tendency would be to stay out of the solution.
Reduction potential of Ag+/Ag= 0.8 is higher than Cu2+/Cu= 0.34 thus, copper sulphate can be stored in silver vessel.