The correct answer is option 'C', α-ketoglutaric acid.

The tricarboxylic acid (TCA) cycle, also known as the citric acid cycle or Krebs cycle, is a series of chemical reactions that occur in the mitochondria of cells. The TCA cycle is an important metabolic pathway that generates energy through the oxidation of acetyl-CoA, a product of carbohydrate, fat, and protein metabolism.

The TCA cycle begins with the entry of acetyl-CoA, which combines with oxaloacetate to form citric acid. Citric acid is a tricarboxylic acid and is formed by the condensation of acetyl-CoA and oxaloacetate. However, citric acid is not the first dicarboxylic acid formed during the TCA cycle.

The first dicarboxylic acid formed during the TCA cycle is α-ketoglutaric acid. Here is the step-by-step breakdown of the TCA cycle and the formation of α-ketoglutaric acid:

1. Acetyl-CoA combines with oxaloacetate to form citric acid.
2. Citric acid undergoes a series of enzymatic reactions, resulting in the release of two molecules of carbon dioxide and the regeneration of oxaloacetate.
3. During these reactions, citric acid is converted to its isomer isocitric acid.
4. Isocitric acid is then oxidized and decarboxylated to form α-ketoglutaric acid. This reaction is catalyzed by the enzyme isocitrate dehydrogenase.
5. α-ketoglutaric acid then enters a series of reactions, including decarboxylation and oxidation, which generate energy in the form of NADH and FADH2. These reactions also regenerate oxaloacetate, allowing the cycle to continue.

In summary, the first dicarboxylic acid formed during the TCA cycle is α-ketoglutaric acid. It is formed from the oxidation and decarboxylation of isocitric acid. The TCA cycle is an essential pathway in cellular metabolism, as it generates energy and provides intermediates for various biosynthetic processes.

During the TCA (tricarboxylic acid or Krebs or citric acid) cycle, the first step where a carbon group is lost as carbon dioxide in a decarboxylation reaction is the compound having 5 carbon known as alpha-ketoglutaric acid with the help of the enzyme isocitrate dehydrogenase.