The remaining energy from glucose molecules, after some is used to produce ATP during glycolysis, is primarily released through the Electron Transport Chain (ETC). This occurs during cellular respiration, specifically in the latter stages which involve the Krebs cycle and oxidative phosphorylation in the mitochondria. Additionally, carbon dioxide is produced as a waste product during the process, but the correct focus for energy release is the ETC.
So the correct response is: Electron Transport Chain.