Carboxylation is the correct answer.

Calvin cycle, also known as the light-independent reactions, is a series of biochemical reactions that take place in the stroma of the chloroplasts in plants. It is responsible for converting carbon dioxide (CO2) into glucose, a process known as carbon fixation.

The Calvin cycle consists of several steps, each catalyzed by a specific enzyme. One of the key enzymes involved in the Calvin cycle is RuBP carboxylase, also known as Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase). This enzyme catalyzes the carboxylation reaction, which is the first step in the Calvin cycle.

Explanation of the carboxylation reaction:

1. Ribulose-1,5-bisphosphate (RuBP), a five-carbon molecule, combines with carbon dioxide (CO2) in the presence of RuBP carboxylase.

2. The enzyme catalyzes the addition of a CO2 molecule to RuBP, resulting in the formation of an unstable six-carbon intermediate compound.

3. This intermediate compound immediately breaks down into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound.

The overall reaction can be represented as follows:
RuBP + CO2 → 2 × 3-PGA

Significance of the carboxylation reaction:

1. The carboxylation reaction is essential for carbon fixation, as it incorporates carbon dioxide from the atmosphere into an organic molecule.

2. The resulting 3-PGA molecules are then used in subsequent reactions of the Calvin cycle to produce glucose and other carbohydrate molecules.

3. The regeneration of RuBP, which is necessary for the continuation of the Calvin cycle, is also dependent on the carboxylation reaction.

In summary, the carboxylation reaction catalyzed by RuBP carboxylase is a crucial step in the Calvin cycle. It allows for the fixation of carbon dioxide and the subsequent production of glucose and other carbohydrates, providing energy and organic compounds for the plant.