The correct answer to this question is Citric acid cycle → NADH → electron transport chain → oxygen. The mitochondrion is an organelle, which has a double membrane. Most eukaryotic organisms have mitochondrion. It is known as the powerhouse of the cell because it creates most of the cells ATP, which is adenosine triphosphate.
ATP in the mitochondrion is produced from the citric acid cycle and fatty acid oxidations. In the pathway, the electron transport chain is the transfer of donor electrons to electron acceptors. This transfer is done through redox reactions. The chain also allows for the transfer of protons across a membrane.