The correct answer is A. Coke.
Coke is a solid fuel made from coal that is heated to high temperatures in the absence of air. This process, called destructive distillation, removes the volatile components of coal, leaving behind a solid residue that is high in carbon. Coke is used as a reducing agent in metallurgy, where it reacts with oxygen to form carbon monoxide. Carbon monoxide is a strong reducing agent, and it is used to extract metals from their ores.
Carbon black is a fine black powder that is produced by incomplete combustion of carbon-containing materials, such as coal, oil, or natural gas. Carbon black is used in a variety of applications, including tires, plastics, and printing inks. It is not a reducing agent.
Charcoal is a solid fuel made from wood or other organic materials that have been heated to high temperatures in the absence of air. Charcoal is used as a fuel in barbecues and fireplaces. It is not a reducing agent.
Graphite is a form of carbon that is made up of layers of carbon atoms arranged in a hexagonal lattice. Graphite is a good conductor of electricity and heat, and it is used in a variety of applications, including batteries, lubricants, and electrodes. It is not a reducing agent.