The correct option is D Oxaloacetic acid
CAM cycle or Crassulacean Acid Metabolism is noticed in the succulent plants like Agave, Opuntia growing under xeric conditions. These plants are called CAM plants.
The stomata of these plants are open during the night and closed during the day to prevent excessive transpiration.
CAM cycle is used for the fixation of atmospheric carbon dioxide in the dark
During the acidification phase of the CAM cycle, the carbon dioxide from the atmosphere is accepted by phosphoenolpyruvate (PEP).
This reaction is catalysed by the enzyme PEP carboxylase and results in the formation of the first stable compound oxaloacetic acid (OAA).