The correct option is A Carbon monoxide
Reducing agents are those substances which reduce other substances (addition of hydrogen or removal of oxygen) and themselves get oxidised (addition of oxygen or removal of hydrogen).
Fe2O3+3CO→2Fe+3CO2
In the above reaction, carbon monoxide (CO) acts as a reducing agent as it reduces ferric(III)oxide(Fe2O3) to form iron (Fe) and carbon dioxide (CO2).