in photorespiration, RuBisCO binds with O2 instead of CO2. Consequently, the substrate RuBP (Ribulose bisphosphate) is not converted into 2 molecules of PGA (3-C) as in the Calvin cycle. Instead, it forms one molecule of phosphoglycerate (a 3-Carbon compound) and one molecule of phosphoglycolate (a 2-Carbon compound) . Option 1 is the correct choice as it reflects the formation of a single 3-C molecule (PGA) in this pathway, distinguishing it from the carboxylation pathway which produces two.