Question

what do geologists use index fossils for?
dating of rock layers
identifying the type of rock in a rock layer
tracking migration of prehistoric species
providing evidence of climate change

Explanation:

Index fossils are used to identify the type of rock layer. Index fossils are fossils of organisms that were widespread but existed for a relatively short period of time. By identifying these fossils in a rock layer, geologists can determine the relative age of that rock layer. This is because if a particular index fossil is found in a rock layer, it indicates that the rock layer was formed during the time when that organism was alive.

Answer:

identifying the type of rock in a rock layer