Question

what determines the length of a geologic time period?
fossil record
material density
meteors and rocks
radioactive materials

Explanation:

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

Answer:

fossil record