Answer:
I. Ultimately transferred to O2 after several other transfer reactions
Step-by-step explanation:
Electrons form glucose oxidation are first taken out from glucose in glycolysis, by NAD+, that is reduced to NADH.
Also NADH is produced when pyruvate (glycolysis product) is decarboxylated and oxidized to acetyl-coenzyme A, previous to enter the Krebs cycle.
In the Krebs cycle, electrons form oxidation of citric acid first, and alpha keto-glutarate later are taken by NAD+, and stored as NADH. Steps later also FAD accepts electrons to produce FADH2. These molecules will deliver these electrons to a respiratory chain, where they will travel until be accepted by O2 to form H2O (some protons are also necessary).