Step 1: The Great Oxidation Event.
The Great Oxidation Event, which occurred around 2.4 billion years ago, was the first major rise in atmospheric oxygen. This event was driven by the activity of cyanobacteria, which performed oxygenic photosynthesis, releasing oxygen into the atmosphere as a byproduct.
Step 2: Explanation of the other options.
Fungi, plants, and viruses did not play a major role in the increase of oxygen in Earth's atmosphere during this period.
Cyanobacteria are the organisms responsible for the Great Oxidation Event due to their ability to carry out oxygenic photosynthesis.