Let's break down how molecular oxygen (O2) is reduced step-by-step: Adding one electron to O2 gives the superoxide ion: O2+e−⟶O2∙− (This is a one-electron reduction.) Adding a second electron gives the peroxide ion: O2+2e−⟶O22− (This is a two-electron reduction.) When O2 undergoes a four-electron reduction, the process is: O2+4e−⟶2O2− Here, each oxygen atom ends up as an oxide ion ( O2− ). So, the four-electron reduced form of O2 is oxide, which corresponds to Option C .