To potentially increase the rate of cellular respiration in the goldfish, the most effective action would be increasing the amount of oxygen in the tank. Cellular respiration in aquatic animals like goldfish relies on oxygen to convert food into energy. By ensuring that there is more oxygen available, you can facilitate a higher rate of cellular respiration.
Increasing the amount of carbon dioxide could have negative effects as it could lead to lower pH and harm the fish. Reducing the amount of food could decrease energy availability, and reducing sunlight would typically not have a direct effect on cellular respiration itself, as goldfish do not rely on photosynthesis. Thus, increasing oxygen is the best choice for enhancing cellular respiration.