When cyanobacteria first appeared on earth, the earth's atmosphere contained little oxygen. cyanobacteria are thought to be responsible for bringing the oxygen in the earth's atmosphere to present levels. this appearance of oxygen and the beginning of aerobic life was made possible by the development of:

The correct answer is - photosystem II.

Explanation:

Photosystem II is a specialized protein complex that traps the light energy from the sunlight and utilizes it to transfer electrons to Pq from water, which ultimately helps in the production of oxygen.

As we know photosystem II helps in production and cyanobacteria were the primitive organism it is considered as after the development of the photosystem II, cyanobacteria begin the aerobic life.

Thus, the correct answer is - Photosystem II.