The correct answer is option 'D' - Oxaloacetic acid.

CO2 fixation is an important process in plants, where carbon dioxide is converted into organic compounds through a series of biochemical reactions. In sorghum, the first stable product of CO2 fixation is oxaloacetic acid. Let's understand the process in detail:

1. Introduction to CO2 fixation:
- CO2 fixation is the process by which carbon dioxide is incorporated into organic molecules.
- This process occurs in plants during photosynthesis, specifically in the light-independent reactions or the Calvin cycle.

2. The Calvin cycle:
- The Calvin cycle is a series of enzymatic reactions that occur in the chloroplast stroma.
- It utilizes the energy from ATP and NADPH, produced in the light-dependent reactions, to convert carbon dioxide into organic compounds.

3. Steps of the Calvin cycle:
a) Carbon dioxide fixation:
- The first step in the Calvin cycle is the fixation of carbon dioxide.
- In this step, the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the reaction between carbon dioxide and ribulose-1,5-bisphosphate (RuBP).
- This reaction produces a 6-carbon intermediate, which immediately splits into two molecules of 3-phosphoglyceric acid (3-PGA).

b) Reduction and carbohydrate synthesis:
- In the next step, ATP and NADPH from the light-dependent reactions are utilized to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).
- One molecule of G3P is used to regenerate RuBP, while the remaining G3P molecules can be further converted into glucose and other carbohydrates.

4. The first stable product:
- During the carbon dioxide fixation step, the first stable product formed in sorghum is oxaloacetic acid.
- Oxaloacetic acid is a 4-carbon compound that plays a crucial role in various metabolic pathways, including the Krebs cycle or citric acid cycle.

5. Significance of oxaloacetic acid:
- Oxaloacetic acid serves as an important precursor molecule for the synthesis of other organic compounds.
- It can be converted into malate, aspartate, or other amino acids.
- It is also involved in the formation of glucose through gluconeogenesis.

In conclusion, the first stable product of CO2 fixation in sorghum is oxaloacetic acid. This compound serves as a starting point for various metabolic pathways and plays a crucial role in the synthesis of organic compounds.