What is waiting at the bottom of the electron transport chain to create water?

Oxygen

Explanation:

Oxygen is the final electron acceptor at the end of the electron transport chain. This is why oxygen is required for aerobic respiration. If oxygen is not present, the electron transport chain will stop running and ATP won't be produced to provide energy for the cell to use.

During the electron transport chain, electrons are moved from a higher energy level to a lower energy level, releasing energy. At the end of the electron transport chain, electrons are transferred to oxygen (O2), which splits and takes up H + to form water.