what determines the length of a geologic time period?\nfossil record\nmaterial density\nmeteors and…

Brief Explanations:

Geologic time periods are defined based on significant changes in the fossil record, which show the evolution and extinction of species over time. Material density, meteors and rocks, and radioactive materials are not the primary determinants of geologic time - period lengths. Radioactive materials are used for dating rocks but not for defining time - periods.

Answer:

fossil record