Carbon Dioxide Fixation:
Carbon dioxide fixation is the process by which carbon dioxide (CO2) is converted into organic compounds during photosynthesis. It occurs in the Calvin cycle, which is the second stage of photosynthesis. The fixation of carbon dioxide is essential for the production of glucose and other organic molecules.

Ribulose-1,5-bisphosphate (RuBP):
RuBP is a 5-carbon molecule that acts as a starting material in the Calvin cycle. It combines with carbon dioxide in the presence of an enzyme called RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) to form an unstable 6-carbon compound.

Formation of 3-phosphoglycerate (3PGA):
3 molecules of 2-phosphoglycerate (2PGA) are formed:
The unstable 6-carbon compound formed by the combination of RuBP and CO2 immediately breaks down into two 3-carbon molecules called 3-phosphoglycerate (3PGA). This is achieved through a process known as carbon dioxide fixation.

Role of RuBisCO:
RuBisCO combines with CO2:
RuBisCO is the enzyme responsible for catalyzing the reaction between RuBP and CO2. It combines with carbon dioxide and facilitates its attachment to the RuBP molecule. This process is known as carboxylation.

RuBP acts as a catalyst:
Although RuBisCO is the catalyst for the reaction, RuBP itself does not act as a catalyst. It is a reactant that reacts with carbon dioxide to form an unstable compound, which is then converted into 3PGA. However, RuBP is essential for the regeneration of RuBisCO during the Calvin cycle.

Summary:
- Carbon dioxide fixation is the process by which carbon dioxide is converted into organic compounds during photosynthesis.
- Ribulose-1,5-bisphosphate (RuBP) is a 5-carbon molecule that combines with carbon dioxide in the presence of the enzyme RuBisCO.
- The combination of RuBP and CO2 forms an unstable 6-carbon compound, which immediately breaks down into two 3-carbon molecules called 3-phosphoglycerate (3PGA).
- RuBisCO combines with carbon dioxide and acts as a catalyst for the reaction, while RuBP is a reactant that participates in the formation of 3PGA.
- Option C is correct because RuBP and CO2 give rise to 3PGA during carbon dioxide fixation.