Metals like mercury and copper can be extracted simply by heating their respective sulphide ores in air. This is an example of:
Correct: C
Less reactive metals like mercury (from Cinnabar, HgS) and copper (from Copper glance, Cu2S) can be extracted by heating their sulphide ores in air. A part of the sulphide ore gets oxidized to its oxide, and then this oxide reacts with the remaining sulphide ore to produce the metal and sulphur dioxide. This process, where the ore reduces itself without an external reducing agent, is known as auto-reduction or self-reduction.