Final answer:
Under normal atmospheric conditions with high O₂ and low CO₂, the enzyme Rubisco most likely produces 2-phosphoglycolate through photorespiration.
Step-by-step explanation:
The most likely product of the reaction catalyzed by Rubisco under normal atmospheric conditions, where O₂ is abundant and CO₂ is low, is 2-phosphoglycolate. Under such conditions, Rubisco favors an oxygenase reaction leading to photorespiration, producing 2-phosphoglycolate instead of 3-phosphoglycerate which is normally formed during carbon fixation when CO₂ concentration is higher.