Based on the fundamental principles of photosynthesis, the best inference is:
A. The closer the light source, the higher the amount of oxygen gas produced.
Photosynthesis occurs in plants when they take in carbon dioxide and water, using light energy (usually from the sun) to convert these into glucose and oxygen as byproducts. Generally, the intensity of light affects the rate of photosynthesis: the closer a light source is, the more light energy reaches the plant, which can increase the rate of photosynthesis and therefore the production of oxygen gas.
Options B, C, and D contradict the expected relationship between light intensity and photosynthetic activity.