Final answer:
In CAM plants, carbon dioxide is bonded to PEP to form oxaloacetate, which is enabled by the enzyme PEP carboxylase.
Step-by-step explanation:
In CAM (Crassulacean Acid Metabolism) plants, CO₂ is bonded to PEP (phosphoenolpyruvate) to form oxaloacetate. This is a two-step process where first, PEP carboxylase catalyzes the fixation of CO₂ to PEP, resulting in oxaloacetate, a four-carbon compound. This process differs from the carbon fixation in C3 plants, where RuBisCO catalyzes the reaction between CO₂ and RuBP (ribulose bisphosphate) to form 3-PGA (3-phospho glyceric acid). The correct answer to the student's question is option (1) PEP, oxaloacetate.