The correct option is B Oxaloacetic acid
C-4 plants and CAM plants follow the Hatch-Slack pathway and the Crassulacean acid metabolism pathway, respectively.
The first stable compound formed during these pathways is Oxaloacetate.
This compound is formed by the addition of carbon dioxide to phosphoenolpyruvate in the presence of the enzyme phosphoenolpyruvate carboxylase.