Question

how do index fossils help scientists?
they aid in the study of ranges of different organisms.
they advance understanding of the origin of life on earth.
they aid in comparison of rock layers at distant locations.
they advance understanding of rock type at the time the organism lived.

Explanation:

Index fossils are fossils that are used to define and identify geologic periods (or faunal stages). They are widespread geographically and existed for a short period of time. By identifying index fossils in rock layers, scientists can compare the ages of rock layers at different locations. This helps in correlating rock layers across vast distances.

Answer:

They aid in comparison of rock layers at distant locations.