Answer:
(a)oxygen
Explanation:
In mitochondria, most of the ATP of non-photosynthetic eukaryotic cells is produced. They metabolize acetyl coenzyme A through the enzymatic cycle of citric acid, giving CO2 and NADH as products. It is the NADH that gives electrons to a chain of electron transporters that is located in the inner membrane.
These electrons pass from one transporter to another arriving as the last step to O2, resulting H2O. This electron transport is coupled to the transport of protons from the matrix to the intermembrane space. It is this proton gradient that allows the synthesis of ATP thanks to ATP synthase. By joining phosphate to ADP and by using oxygen as the final electron acceptor, this process is called oxidative phosphorylation.
