Step 1: Understand the process of smelting.
Smelting is a metallurgical process where metal oxides are reduced to pure metal. It generally requires a reducing agent, which removes oxygen from the metal oxide.
Step 2: Identify the correct reducing agent.
- Aluminum (Al) and magnesium (Mg) can be used in certain reactions, but carbon (C) is the most common reducing agent for smelting.
- Carbon reacts with oxygen in metal oxides to produce carbon dioxide and pure metal.
Step 3: Apply to the options.
Carbon (C) is widely used in smelting processes, making it the correct choice.
Final Answer:
\[
\boxed{\text{C}}
\]