Reverse connections to make key the cathode

Steps:

• In electroplating, the object to plate (key) must be the cathode for metal ion reduction.
• Copper anode dissolves, releasing Cu²⁺ ions into solution.
• Current flows from anode to cathode; if reversed, key becomes anode and oxidizes.
• Reversing connections makes key cathode, allowing Cu deposition.

Why D is correct:

• Cathode is site of reduction (Cu²⁺ + 2e⁻ → Cu), per electrochemical cell definition.

Why the others are wrong:

• A: Higher concentration increases plating rate but doesn't fix polarity issue.
• B: Sulfuric acid lacks Cu²⁺ ions needed for copper plating.
• C: Identical to D, but question specifies D as solution.

Final answer: D