D

Step 1: Understand the process of electroplating.
Electrodeposition (or electroplating) involves using an electrical current to deposit a layer of metal onto a conductive surface. In the case of gold plating on copper, we need a solution containing gold ions.
Step 2: Identify the options.
- Option A (Copper sulphate) contains copper ions, which are not required for gold plating.
- Option B (Copper chloride) also contains copper ions, so it's not suitable.
- Option C (Potassium cyanide) is used in various chemical processes but does not provide gold ions directly.
- Option D (Potassium aurocyanide) contains gold ions and is specifically used in gold plating processes.
Therefore, the correct option is D, as it is the only one that provides the necessary gold ions for the plating process.