Final answer:
Oxygen is the substance that bonds to hydrogen at the end of the electron transport chain, forming water and allowing electron flow necessary for ATP production.
Step-by-step explanation:
At the end of the electron transport chain, the substance that bonds to hydrogen is oxygen. As electrons move through the electron transport chain, they are passed from one protein complex to another until they are finally accepted by oxygen, the terminal electron acceptor. This oxygen then combines with hydrogen ions (H+) to form water (H2O). This step is crucial for the production of ATP in aerobic respiration, as it maintains the flow of electrons through the chain and allows for the establishment of an electrochemical gradient that drives the synthesis of ATP via oxidative phosphorylation.