Final answer:
Oxygen entered Earth's system through the activity of cyanobacteria producing oxygen via photosynthesis, with fossils of these bacteria dating back about 3.5 billion years. Dinosaurs emerged much later and humans appeared only around 200,000 years ago, with the last ice age ending around 11,700 years ago. Therefore, the correct order is oxygen, oldest fossils, first dinosaurs, dinosaurs total, first humans, and end of the ice age.
Step-by-step explanation:
The chronological order of events from the question can be understood by looking at Earth's geologic history. Oxygen first entered Earth's system through the activity of photosynthetic bacteria like cyanobacteria. These microbes evolved early in Earth's history and started releasing oxygen as a by-product of photosynthesis. This process led to the oxygenation of the atmosphere. Cyanobacteria are thought to have appeared within one billion years of Earth's formation. The oldest known fossils are of stromatolites, which are colonial photosynthetic cyanobacteria, dating back approximately 3.5 billion years.
The first dinosaurs appeared much later, during the Mesozoic Era, around 230 million years ago. Dinosaurs then dominated the planet for about 165 million years until their mass extinction approximately 65 million years ago. The first anatomically modern humans emerged around 200,000 years ago. The last ice age ended around 11,700 years ago, marking the beginning of the current geological epoch, the Holocene.
So, the correct chronological order is: Oxygen enters Earth's system, oldest fossils, first dinosaurs, dinosaurs total, first humans, ice age ends, which can be represented as option (a).