Final answer:
The primary criterion for demarcating the divisions of the geologic time scale is major changes recorded in the fossil record, specifically the appearance or disappearance of significant forms of life. Therefore, the correct option is C.
Step-by-step explanation:
The main criterion that scientists use to decide where to place the boundaries between the major divisions of the geologic time scale is C. Major changes that are recorded in the fossil record. This includes significant shifts in the types of fossils found within the geological strata, which often correspond to periods of mass extinction, the diversification of lifeforms, or other notable evolutionary events.
The geologic time scale is an important tool for understanding Earth's history, organizing major geological and biological changes rather than solely by numeric ages. Divisions such as eons, eras, periods, and epochs are based on these substantial changes. Each division reflects a distinct time during which the Earth experienced changes in its climate, geology, and biology.
The Earth formed during the Hadean Eon, and we are currently in the Phanerozoic Eon, within the Cenozoic Era. Boundaries in the geologic time scale, such as the one marking the end of the Cretaceous Period and the start of the Paleogene Period, highlight the mass extinction that wiped out the dinosaurs and many marine species. These changes are well-documented by shifts in the fossil record.