The correct option is C succinic acid to fumaric acid
Krebs cycle can be defined as a stepwise, oxidative and cyclic degradation of acetyl CoA derived from the end product of glycolysis i.e. pyruvate.
In the 6th step of Krebs cycle, oxidation of succinate occurs. Succinate undergoes dehydrogenation (removal of hydrogen) to form fumaric acid (also known as fumarate) with the help of a membrane based enzyme succinate dehydrogenase. FADH2 is produced in this step.