RuBisCO catalyses the addition of CO₂ to:

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the primary enzyme involved in the first major step of the **Calvin Cycle** during photosynthesis. 1. **Carbon Fixation**: RuBisCO catalyzes the fixation of atmospheric $CO_2$ into organic matter. The molecule that accepts the $CO_2$ is a 5-carbon sugar called **Ribulose-1,5-bisphosphate (RuBP)**. 2. **The Reaction**: The reaction can be summarized as: $$CO_2 + \text{RuBP} \xrightarrow{\text{RuBisCO}} [\text{6C intermediate}] \rightarrow 2 \times \text{3-PGA}$$ 3. **Analysis of Options**: - **G3P**: A 3-carbon sugar produced later in the cycle after reduction of 3-PGA. - **PEP**: The $CO_2$ acceptor in $C_4$ plants, catalyzed by PEP carboxylase, not RuBisCO. - **3-PGA**: The first stable 3-carbon product formed *after* $CO_2$ is added to RuBP. Since RuBisCO specifically binds $CO_2$ to the 5-carbon RuBP, the correct answer is **RuBP**.