Final answer:
The correct answer is option a. Geological eras are primarily separated by mass extinctions and major speciation events as evidenced in the geologic record, which can include catastrophic biological or geologic occurrences.
Step-by-step explanation:
Markers used to separate geological eras are often defined by significant changes in the diversity of life on Earth, which are usually marked by mass extinctions or major speciation events. Eras are distinguished in the geologic record where there is evidence of a dramatic loss of species, followed by the eventual emergence of new species.
Such transitions are identifiable in rock strata and can correspond with catastrophic geologic or biological events, including volcanic eruptions and meteor impacts, which can bring about mass extinctions that end an era. For instance, the end of the Paleozoic Era is defined by the largest mass extinction in Earth's history, while the end of the Mesozoic Era is marked by the extinction of dinosaurs followed by the rise of mammals.
A clear example provided is the boundary between the Triassic and Jurassic periods, around 201.3 million years ago, which corresponds to a mass extinction event possibly related to significant volcanic activity. The geologic time scale is a useful tool in representing these significant transitions, often featuring 'golden spikes' that indicate specific places used to define the age boundaries. These golden spikes are based on evidence from various sources, including outcrop stratigraphy, as well as the fossil record which shows the changes in species before and after the extinction events.