Decarboxylation is a chemical reaction in which a carboxyl group (-COOH) is removed from a molecule, resulting in the formation of a carbon dioxide molecule (CO2). Ketoglutaric acid, also known as alpha-ketoglutaric acid, is a key intermediate in the citric acid cycle (also known as the Krebs cycle or TCA cycle) and plays a crucial role in energy production in cells.

In order to understand the steps of decarboxylation leading to the formation of ketoglutaric acid, let's break it down:

1. Initial molecule: The initial molecule involved in the decarboxylation leading to ketoglutaric acid is isocitric acid. Isocitric acid is an intermediate in the citric acid cycle and has three carboxyl groups (-COOH) attached to it.

2. First decarboxylation: Isocitric acid undergoes a decarboxylation reaction catalyzed by the enzyme isocitrate dehydrogenase. In this reaction, the carboxyl group (-COOH) on the middle carbon of isocitric acid is removed, resulting in the formation of alpha-ketoglutaric acid. This is the first step of decarboxylation.

3. Formation of alpha-ketoglutaric acid: The removal of the carboxyl group from isocitric acid results in the formation of an intermediate molecule called oxalosuccinic acid. Oxalosuccinic acid then undergoes a rearrangement reaction, known as an intramolecular rearrangement, which leads to the formation of alpha-ketoglutaric acid.

Therefore, there are two steps of decarboxylation involved in the formation of ketoglutaric acid. The first decarboxylation occurs during the conversion of isocitric acid to oxalosuccinic acid, and the second decarboxylation occurs during the rearrangement of oxalosuccinic acid to alpha-ketoglutaric acid.

Hence, the correct answer is option C) Two.