The enzyme required for carboxylation in the Calvin cycle is :

The Calvin cycle is the set of biochemical reactions that plants use to fix atmospheric carbon dioxide (CO₂) into organic molecules during photosynthesis.

The first and critical step of the cycle is the carboxylation of ribulose‑1,5‑bisphosphate (RuBP). This reaction adds CO₂ to RuBP, producing an unstable six‑carbon intermediate that immediately splits into two molecules of 3‑phosphoglycerate (3‑PGA).

The enzyme that catalyzes this carboxylation is **RuBP carboxylase–oxygenase**, commonly known as Rubisco. Rubisco has two activities:

• Carboxylation: RuBP + CO₂ → 2 3‑PGA (the desired fixation reaction).
• Oxygenation (when O₂ competes with CO₂): RuBP + O₂ → 3‑PGA + 2‑phosphoglycolate (photorespiration).

This dual activity gives the enzyme its name “carboxylase–oxygenase.”

None of the other listed enzymes participate in the Calvin cycle:

• PEP carboxylase functions in C₄ and CAM plants to initially fix CO₂ into oxaloacetate, not in the Calvin cycle.
• Carboxypeptidase is a proteolytic enzyme that removes terminal amino acids from proteins.
• Hexokinase phosphorylates glucose to glucose‑6‑phosphate in glycolysis, not in photosynthetic carbon fixation.

Therefore, the enzyme required for the carboxylation step of the Calvin cycle is **RuBP carboxylase–oxygenase**.