The first stable product of CO₂ fixation in sorghum is :

The correct option is Oxaloacetic acid.

Step-by-Step Explanation:

1. Understanding the type of plant: Sorghum is a C₄ plant. Unlike C₃ plants, C₄ plants have adapted to dry, hot environments and utilize a specialized pathway for carbon fixation (the Hatch-Slack pathway) to minimize photorespiration.

2. Carbon Fixation in C₄ Plants: In the mesophyll cells of C₄ plants like sorghum, the initial fixation of carbon dioxide (${\mathrm{CO}}_2$) is catalyzed by the enzyme PEP carboxylase (Phosphoenolpyruvate carboxylase).

3. The Initial Product: The primary acceptor of ${\mathrm{CO}}_2$ is phosphoenolpyruvate (PEP), which is a 3-carbon compound. When PEP combines with ${\mathrm{CO}}_2$, it forms a 4-carbon organic acid called oxaloacetic acid (OAA).

4. Conclusion: Since oxaloacetic acid is the very first stable intermediate compound formed after ${\mathrm{CO}}_2$ fixation in this pathway, it is the first stable product of ${\mathrm{CO}}_2$ fixation in sorghum.