Aerobic and anaerobic respiration are basically two means of utilizing food for energy. The main difference between each is the utilization of oxygen. The oxygen is available in abundance in aerobic respiration, so the glucose is fully broken down producing a larger yield of energy. On the other hand, in anaerobic respiration, there is no oxygen to accept the electrons.
Hence, the glucose is only partially broken down to pyruvate, which is then converted to lactic acid. This yields lower amount of energy but acts as an emergency source when there is unavailability of oxygen, such as during vigorous exercise.
Only oxygen because exercise is not a molecule. Another thing is that anaerobic specifically means the absence of oxygen while aerobic means the presence of oxygen. Therefore oxygen is the only correct answer. Exercise is not a reactant of photosynthesis or cellular respiration and has nothing to do with this question.