Answer:
1 Meter.
Step-by-step explanation:
This is because the production of oxygen from the set up was due to the splitting of water into H+,e- and O2, by sunlight ,(Photolysis).
The sunlight was replaced by the light source.
Therefore, the closer the intensity of light to water, the more the amount of light available to breakdown water to produce O2,Hence, 1 meter source produces more oxygen than 3 meters