Index fossils are a relatively rare and difficult to find. b fossils that are found in limestone. c fossils of species that still exist today. d imprints of organisms known to have lived during a specific time period.

D. imprints of organisms known to have lived during a specific time period

Explanation:

The index fossils are very useful to the geologists. These fossils are only found in specific time period. They are also found on lot of locations around the globe, and they are in big numbers. Because of these characteristics, the index fossils are excellent for determining the relative age of rock layers, or other fossils, as they offer the basic information that are needed for this. This type of fossils tend to be small or medium sized ones, as usually the larger organisms do not have very large populations.

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