Hint: All living cells can synthesize and break down molecules (carbohydrates, proteins, etc.) to carry out various cellular functions. This breakdown and synthesis of a molecule are possible due to several chemical reactions that occur in the cell. When molecules are synthesized from simpler molecules it is called anabolism. The breakdown of complex molecules is called catabolism. Metabolism involves both these processes.
Lipids, carbohydrates, and proteins are macromolecules that can be broken down into simpler products. This breakdown releases energy. One of the products that all three macromolecules produce, is Acetyl CoA. Break down of the carbon skeleton of lipids, carbohydrates, and protein gives this molecule.
Krebs cycle is also known as the citric acid cycle or tricarboxylic acid cycle (TCA). Organisms that thrive in aerobic conditions can carry out this cycle. In Eukaryotes, the Krebs cycle takes place in the mitochondria.
A 4-C compound, Oxaloacetate, present in the mitochondria accepts the Acetyl CoA to give a six-carbon compound that is Citrate. Every reaction in this cycle is catalyzed by different enzymes. Citrate is converted to Isocitrate. This Isocitrate is acted upon by the enzyme Isocitrate dehydrogenase.
This oxidation reaction gives an unstable six-carbon compound called Oxalosuccinate (oxalosuccinic acid). As this compound is unstable it gets converted to $\alpha $- Ketoglutarate which is a five-carbon compound. This cycle continues and the end product formed is oxaloacetate.
Hence, the correct answer is option (D).
Note: Many bacterial cells also carry out the Krebs cycle. In bacterial cells, this cycle takes place in the cytosol as bacteria lack the cell organelle mitochondria. Apart from the energy-rich molecule (GTP that gets converted to ATP) produced; the intermediates of the Krebs cycle can also be used for other biosynthetic processes.