Match reactions P: $R-H+O_2+2H^++2e^-\to R-OH+H_2O$; Q: $O_2+4e^-+8H^+\to2H_2O+4H^+$; R: $2H_2O_2\to2H_2O+O_2$; S: $NH_2CH_2CO_2H\to NH_2CH(CH_2OH)CO_2H$, with I coenzyme B₁₂, II cytochrome P-450, III cytochrome c oxidase, IV catalase.
P is monooxygenation by cytochrome P-450, II. Q is oxygen reduction coupled with proton translocation by cytochrome c oxidase, III. R is peroxide disproportionation by catalase, IV. These three unambiguous matches select C. The source then assigns S to coenzyme B₁₂ by elimination; however, the printed glycine-to-serine conversion adds a carbon unit without showing its donor and is not a complete standard B₁₂ rearrangement. The historical wording and grading are preserved with that limitation, rather than asserting the incomplete equation is a balanced B₁₂ reaction.
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