Thermal reduction is a process used to extract metals from their ores. In this process, the metal oxide is heated in the presence of a reducing agent, which causes the reduction of the metal oxide to the pure metal. Among the given options, copper is extracted by the thermal reduction process.

Here is a detailed explanation of why copper is extracted by the thermal reduction process:

1. Copper extraction process:
- Copper is mainly extracted from copper sulfide ores, such as chalcopyrite (CuFeS2) and bornite (Cu5FeS4). These ores are first crushed and then concentrated by froth flotation, which separates the copper sulfide minerals from the gangue (unwanted materials).
- The concentrated ore is then roasted in a furnace to convert the copper sulfide minerals into copper oxide (CuO). The chemical reaction involved in the roasting process is as follows:
2CuFeS2 + 4O2 → 2CuO + 2FeO + 4SO2
- The copper oxide obtained from the roasting process is then subjected to thermal reduction to obtain pure copper metal.

2. Thermal reduction of copper oxide:
- The thermal reduction of copper oxide is carried out in a furnace at high temperatures. The reducing agent used in this process is carbon (in the form of coke or coal) or hydrogen gas.
- The chemical reaction involved in the thermal reduction of copper oxide with carbon is as follows:
CuO + C → Cu + CO
- In this reaction, carbon (C) acts as the reducing agent and reduces copper oxide (CuO) to copper (Cu) while forming carbon monoxide (CO) as a byproduct.
- The copper metal obtained from the reduction process is then further refined through various methods to obtain pure copper.

In summary, copper is extracted by the thermal reduction process, where copper oxide is heated in the presence of a reducing agent (such as carbon) to obtain pure copper metal. This process is commonly used in the extraction of copper from copper sulfide ores.