Correct choice is (d) Citric acid
The explanation is: The first product of TriCarboxylic Acid cycle is citric acid. Therefore, it is also called a citric acid cycle or Krebs cycle. The cycle is primarily regulated by the concentration of ATP, NADH and is controlled by the key enzymes like isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, etc.