The correct option is A Oxaloacetate
Oxaloacetate is the first stable product produced during night time in CAM plants. In these plants, carboxylation of phosphoenolpyruvate takes place to form oxaloacetate which is then reduced to malate. This malate is then stored in the vacuoles. During day time, the vacuolar malic acid is decarboxylated in the cytosol, and the CO2 released enters the Calvin cycle in the chloroplasts.