Final answer:
Electrode design for AC current contains a rare earth thorium ingredient (Option D).
Step-by-step explanation:
The ingredient often used in electrode design for AC current that is a rare earth element is Thorium. Thorium has been utilized in the manufacture of electrodes, particularly in applications where high temperatures are involved. It can be added to tungsten electrodes used in gas tungsten arc welding (GTAW) to enhance the electrode's properties such as its electron emission.
In the context of electrode design, alternatives like Copper, Aluminum, and Tungsten are metals that do not belong to the category of rare earth elements and thus are not typically described as such in this application.
Thus, the correct option is D.