The answer is c. oxygen.
Cellular respiration includes glycolysis, link reaction, Krebs cycle, and electron transport chain (ETC), in that order. The main function of ETC is a production of ATP. In this series of oxidation-reduction reactions, electrons from the previous stages of cellular respiration are taken and transported to the oxygen which is the final acceptor of electrons. As the result, water and ATP are produced.
Although in many regions of the brain—particularly those that give rise to nuclear cell groups—neurons migrate without the benefit of glial guides, migration along radial glial fibers is always seen in regions where cells are organized into layers, like the cerebral cortex, hippocampus, and cerebellum.
