Which organism makes the most of Earth's oxygen?

Answer: Microscopic ocean algae

Microscopic ocean algae ✓ — Correct! Phytoplankton — microscopic algae drifting in the ocean — produce 50-80% of Earth's oxygen. One genus alone, Prochlorococcus, makes about 20% on its own. These cells are so small that 100 of them span a human hair's width, yet collectively they out-produce every forest on land.

Rainforest trees like the Amazon — Not quite. The Amazon is often called 'the lungs of the Earth,' but it's closer to net-neutral on oxygen. Trees release O₂ by day, while respiration (trees, animals, and microbes) consumes nearly the same amount. The net contribution to the atmosphere is close to zero.

Moss and lichen on the tundra — Not quite. Mosses and lichens do photosynthesize, but the tundra has a short growing season and thin vegetation, so its total oxygen output is a tiny slice of the global budget. It's a slow, cold biome, not a factory floor.