The correct answer is C) Aluminum. Aluminum can be cut with the highest operating lathe speed compared to the other metals listed. This is because aluminum has a lower tensile strength and is softer than cast iron, brass, and mild steel, allowing it to be machined at higher speeds without causing excessive wear or damage to the cutting tool. The higher machinability of aluminum also means it generates less heat during the cutting process, further enabling higher lathe speeds. In contrast, the other metals like cast iron, brass, and mild steel have higher tensile strengths and are generally harder, requiring lower lathe speeds to avoid excessive tool wear and maintain the desired surface finish.