The correct answer is option 3: Glycolate. Photorespiration is a metabolic pathway that occurs in plants when there is a high concentration of oxygen and a low concentration of carbon dioxide. In this process, the enzyme RuBisCO, which normally fixes carbon dioxide in the Calvin cycle, instead binds to oxygen. This forms a compound called phosphoglycolate, which is then converted into glycolate.

Glycolate is the substrate of photorespiration because it is the first product formed when RuBisCO binds to oxygen instead of carbon dioxide. It is eventually converted into glyoxylate, which can enter the peroxisome and be further metabolized. This process is energy-consuming and reduces the efficiency of photosynthesis. Therefore, photorespiration is considered to be a wasteful pathway.