Question

which of the following events separate geologic time into eras and periods?
the number of earthquakes occurring at that time
the creation of new ocean ridges
the appearance and disappearance of groups of organisms
the appearance and disappearance of volcanoes

Explanation:

Geologic time is divided into eras and periods based on significant changes in the fossil record. The appearance and disappearance of groups of organisms (fossil evidence) are key. Earthquakes, ocean ridges, and volcanoes don't directly define these time divisions as comprehensively as biological changes.

Answer:

the appearance and disappearance of groups of organisms