The end product of glycolysis is a three-carbon molecule pyruvic acid. Pyruvic acid is first converted to acetyl coenzyme A by a link reaction. This acetyl coenzyme A is a two-carbon molecule. This reaction results in the decarboxylation of the pyruvic acid. The acetyl coenzyme A thus formed enters the Krebs cycle and along with oxaloacetic acid forms a 6 C molecule citric acid. So, the correct option is D.

Introduction
The link between Glycolysis and the Krebs cycle (also known as the Citric Acid Cycle or TCA Cycle) is a critical step in cellular respiration. Understanding this connection is essential for grasping how energy metabolism functions in living organisms.
Role of Glycolysis
- Glycolysis occurs in the cytoplasm, where one glucose molecule is broken down into two molecules of pyruvate.
- This process yields a net gain of 2 ATP and 2 NADH molecules, which are vital energy carriers.
Transition to the Krebs Cycle
- Pyruvate produced in glycolysis cannot directly enter the Krebs cycle.
- Before entering the Krebs cycle, pyruvate undergoes a transformation to form Acetyl-CoA.
Formation of Acetyl-CoA
- Each pyruvate molecule loses one carbon atom in the form of carbon dioxide (decarboxylation).
- The remaining two-carbon fragment combines with coenzyme A to form Acetyl-CoA.
- This process is catalyzed by the enzyme pyruvate dehydrogenase.
Connection to the Krebs Cycle
- Acetyl-CoA is the key substrate that enters the Krebs cycle.
- In the Krebs cycle, Acetyl-CoA is further oxidized, producing NADH, FADH2, and ATP, which are essential for the electron transport chain and energy production.
Conclusion
- The transformation from pyruvate to Acetyl-CoA is crucial as it links glycolysis to the Krebs cycle.
- Thus, the correct answer to the question regarding the link between Glycolysis and the Krebs cycle is indeed option 'D'—Acetyl co-enzyme. This connection is fundamental for efficient energy extraction from glucose.