CO₂ joins the photosynthetic pathway during:

Explanation

The correct answer is the dark reaction, which is also referred to as the Calvin cycle. This is the stage of photosynthesis where carbon dioxide (CO₂) is incorporated into organic molecules, a process known as carbon fixation. This occurs in the stroma of the chloroplasts and does not require light to proceed. The Calvin cycle ultimately leads to the formation of glucose. The other options (Photosystem I, Photosystem II, and Photolysis) all pertain to the light-dependent reactions, which are primarily concerned with capturing light energy to produce ATP and NADPH, and do not directly involve CO₂ assimilation.