Object must be cathode for electroplating Steps:

• Identify setup: Steel key in copper(II) sulfate solution with electrical connections for plating.
• Recall electroplating: Cathode (negative) receives metal ions and gets plated; anode (positive) dissolves.
• Diagnose issue: Key not plating means it's likely connected as anode, repelling Cu²⁺ ions.
• Solution: Swap connections so key becomes cathode, attracting Cu²⁺ ions to deposit copper.

Why D is correct:

• Reversing connections makes the key the cathode, where reduction (Cu²⁺ + 2e⁻ → Cu) occurs per Faraday's laws of electrolysis.

Why the others are wrong:

• A: Higher concentration increases ion availability but won't fix wrong polarity preventing deposition.
• B: Sulfuric acid lacks Cu²⁺ ions, so no copper plating possible; it's for cleaning, not plating.
• C: Identical to D, so redundant and doesn't apply uniquely.

Final answer: D