With reference to the planet Earth, consider the following statements: [Physical Geography]
I. Rain forests produce more oxygen than that produced by oceans.
II. Marine phytoplankton and photosynthetic bacteria produce about 50% of world’s oxygen.
III. Well-oxygenated surface water contains several folds higher oxygen than that in atmospheric air.
Which of the statements given above is/are correct?

The correct option is II only.

Let us analyze each statement step-by-step to understand why this option is correct:

Statement I: Rain forests produce more oxygen than that produced by oceans.
This statement is incorrect. While terrestrial rain forests (like the Amazon) are often called the "lungs of the Earth," they actually produce less net oxygen than the oceans. In fact, most of the oxygen produced by rain forests is consumed by the organisms living within those same ecosystems through respiration and decay. On the other hand, marine organisms (like phytoplankton) produce a vast majority of the Earth's net oxygen.

Statement II: Marine phytoplankton and photosynthetic bacteria produce about 50% of world's oxygen.
This statement is correct. Marine organisms, primarily phytoplankton, algae, and photosynthetic bacteria (such as Prochlorococcus), are responsible for producing at least 50% (and by some estimates up to 70-80%) of the Earth's atmospheric oxygen through photosynthesis. Therefore, statement II is scientifically accurate.

Statement III: Well-oxygenated surface water contains several folds higher oxygen than that in atmospheric air.
This statement is incorrect. The concentration of oxygen in atmospheric air is much higher than that in well-oxygenated water. Atmospheric air contains about 21% oxygen by volume (which is equivalent to approximately 210,000 parts per million or ppm). In contrast, even fully saturated, well-oxygenated surface water contains only about 8 to 10 milligrams of dissolved oxygen per liter at standard temperatures (which is roughly 8 to 10 ppm). Thus, atmospheric air contains tens of thousands of times more oxygen than well-oxygenated water, not the other way around.

Since only statement II is correct, the correct choice is II only.