Final answer:
Hydrogen atoms from NADH end up as part of water after being processed by the electron transport chain during cellular respiration.
Step-by-step explanation:
In the electron transport system, hydrogen atoms removed from NADH ultimately end up as part of water. During the final stages of cellular respiration, the electron transport chain utilizes electrons donated by NADH (and FADH₂). These electrons are passed through a series of complexes, releasing energy that is used to pump protons into the intermembrane space of the mitochondria, which then flow back through ATP synthase to produce ATP.
Electrons from NADH are finally accepted by oxygen, the terminal electron acceptor, forming water. Without oxygen, these electrons cannot be cleared from the system, causing the electron transport chain to halt and preventing the production of ATP. Thus, the correct answer is water, as hydrogen atoms from NADH combine with oxygen to form water at the end of the electron transport chain.