Which compound is formed at the end of the electron transport chain?

At the end of the electron transport chain, the primary compound that is formed is water. This reaction occurs when electrons, after traveling through various carriers in the chain, ultimately combine with oxygen and protons (hydrogen ions). Oxygen acts as the final electron acceptor, and when it interacts with these electrons and protons, it leads to the formation of water.

This process is essential for aerobic respiration, as it helps to maintain the flow of electrons through the electron transport chain, allowing the continual production of ATP, the energy currency of the cell. In the complete oxidation of glucose during cellular respiration, the formation of water is a crucial step that signifies the effective use of oxygen, highlighting the importance of the electron transport chain in energy metabolism.

The other compounds mentioned do not directly result from the electron transport chain's final reactions, which is why they are not the correct answer.