The joining of carbon dioxide to RuBP occurs in the Calvin cycle, which takes place in the stroma of the chloroplast.

Here's why:

* Calvin cycle: This is the primary pathway for carbon fixation in photosynthesis, where carbon dioxide is converted into sugar.

* RuBP (ribulose-1,5-bisphosphate): This is a 5-carbon sugar that acts as the primary carbon dioxide acceptor in the Calvin cycle.

* Stroma: This is the fluid-filled space within the chloroplast where the Calvin cycle takes place.

So, the entire process of carbon dioxide fixation, including the joining of CO2 to RuBP, happens within the stroma of the chloroplast during the Calvin cycle.