1. The problem asks to identify the process where carbon undergoes oxidation number changes from +4 to +2 and back to +4. 2. Oxidation numbers indicate the degree of oxidation or reduction of an element in a compound. 3. In metallurgy, different furnaces facilitate different chemical reactions involving carbon. 4. The electric furnace is typically used for melting and refining metals, not specifically for changing carbon oxidation states in this pattern. 5. The oxygen converter (or basic oxygen furnace) is used in steelmaking where carbon is oxidized from a higher to a lower oxidation state and vice versa during the process. 6. The blast furnace is primarily used for reducing iron ore to iron, where carbon changes oxidation states but not typically in the +4 → +2 → +4 sequence. 7. The Midrex furnace is used for direct reduction of iron ore using natural gas, involving different carbon chemistry. 8. The oxidation number changes +4 → +2 → +4 for carbon are characteristic of the oxygen converter process. Final answer: b) oxygen convertor