The answer is: A.H2O2
I assume the reaction is 2 H2O2 ==> 2 H2O + O2
This reaction could be accelerated by catalase ezyme. The substrate would be hydrogen peroxide(H2O2) which is an reactive oxygen species(ROS). Hydrogen peroxide is dangerous because it could oxidize other subtance and need to broken down to water(H2O) and oxygen(O2) to prevent damage to the cell. Catalase is an important enzyme for this reaction and this enzme could be found in any organism that exposed to oxygen.
