Index fossils are

A. fossils that are found in limestone.

B. relatively rare and difficult to find.

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

D. fossils of species that still exist today.

Answer: C. imprints of organisms known to have lived during a specific time period.

Explanation:

Index fossils are the fossils which are used to compare the age of the other fossils. These fossils can be used for relative comparison and providing the relative age of the fossils. These fossils can be considered to be living in a particular geological time. These fossils exhibit a wide geographical distribution but exhibit a short time duration or specific time of their survival.

These fossils can be used for determining the geological time scale of the organisms or rocks as these allows the correlation of the entities with the strata.

These fossils can be found as imprint fossils over the surface of the rocks as the organisms after death buried inside the heap of earth. Their soft body parts are consumed by the microbes and the hard remains are left. These hard remains leave their impression over the hard surfaces such as rocks.

On the basis of the above description, C. imprints of organisms known to have lived during a specific time period. is the correct option.

C. is your answer. (Index fossils are useful for dating)