The correct option is (a) Solvents, Acid and Gases
For explanation: Clostridium is an obligate anaerobe. It is used as a solvent, acid as well as gas in microbial oil recovery. As solvent it is used to dissolve rocks for better permeability and reduction of oil viscosity, as acid it is used for emulsification and aslo to increase permeability, and as gas it is used to increase pressure and for oil swelling. Hence, Clostridium is used as solvents, acid and gases.