Final answer:
Oxygen is the final electron acceptor that combines with hydrogen ions to form water in the electron transport chain during cellular respiration.
Step-by-step explanation:
The final electron acceptor that combines with H+ to form water (H2O) in the electron transport chain is oxygen. This process occurs during cellular respiration where, in the electron transport chain, the electrons from NADH and FADH2 are passed through a series of electron carriers and finally transferred to oxygen, which then combines with hydrogen ions to form water. Without oxygen, the electron transport chain cannot function as there would be no final electron acceptor, halting the production of ATP.