In the Krebs cycle, [D] Fumaric acid is synthesized. The Krebs cycle, also known as the citric acid cycle, is a central metabolic pathway occurring in the mitochondria. It begins with the condensation of acetyl-CoA and oxaloacetate to form citrate. Through a series of enzyme-catalyzed reactions, citrate is sequentially converted into various intermediate compounds, including fumaric acid. These reactions involve oxidation-reduction reactions and substrate-level phosphorylation, ultimately leading to the regeneration of oxaloacetate to sustain the cycle.

The Krebs cycle plays a crucial role in cellular respiration, serving as a source of high-energy electrons used to generate ATP through oxidative phosphorylation. While pyruvic acid is a product of glycolysis, and lactic acid can be formed during anaerobic conditions, glucose is not synthesized during the Krebs cycle.