In the context of the relationship between photosynthesis and cellular respiration, the labels likely represent the input and output gases involved in these processes.
Photosynthesis takes in carbon dioxide (CO2) and water (H2O) and produces oxygen (O2) and glucose (C6H12O6). On the other hand, cellular respiration uses oxygen (O2) and glucose (C6H12O6) to produce carbon dioxide (CO2) and water (H2O).
Given this information, the best response is:
1-oxygen; 2-carbon dioxide
This indicates that oxygen is produced during photosynthesis (1) and carbon dioxide is produced during cellular respiration (2).