The correct option is II only because marine phytoplankton and photosynthetic bacteria together contribute approximately 50% of the Earth's oxygen through photosynthesis. Statement I is incorrect, as while rain forests are vital for oxygen production, the ocean, particularly through phytoplankton, generates a greater share. Statement III is also inaccurate; well-oxygenated surface water does contain dissolved oxygen, but it is generally at much lower concentrations compared to the oxygen available in the atmosphere, where it is around 21%. Thus, only statement II accurately represents the major sources of oxygen on Earth as reflected in ecological studies and environmental science textbooks.