The first stable product of CO₂ fixation in sorghum is :

The question asks for the first stable product that appears after CO₂ is fixed in sorghum.

Sorghum is a C₄ plant. In C₄ photosynthesis the initial fixation of CO₂ does not occur with the enzyme RuBisCO as in C₃ plants. Instead, CO₂ is first hydrated to bicarbonate (HCO₃^-) and then combined with phosphoenolpyruvate (PEP).

The enzyme that catalyzes this reaction is phosphoenolpyruvate carboxylase (PEPC). The reaction can be written as:

$PEP\; +\; HCO₃⁻\; \to \; Oxaloacetate\; +\; Pi$

Because the product of this carboxylation, oxaloacetate, is a four‑carbon dicarboxylic acid, it is stable enough to be transported out of the mesophyll cell into the bundle‑sheath cell where it later releases CO₂ for the Calvin cycle.

No other compound is formed directly by the CO₂‑fixing step in sorghum. Succinic acid, phosphoglyceric acid, and pyruvic acid are downstream metabolites that appear after further processing of oxaloacetate, but they are not the immediate, first stable product of CO₂ fixation.

Therefore, the correct answer is Oxaloacetic acid.