The correct option is B oxaloacetic 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. Krebs’ cycle starts with condensation of acetyl group (acetyl CoA) with oxaloacetate (4- carbon compound) in the presence of a condensing enzyme citrate synthase to form a tricarboxylic, 6- carbon compound called citric acid. It is the 1st product of Krebs cycle. CoA is liberated. Hence oxalo acetic acid is the 1st member of the TCA cycle.