Trilobites are index fossils because they a. changed slowly through time c. exhibit specific characteristics b. lived over large areas of the world d. all of the above

d. all of the above

Explanation:

Trilobites are an extinct class of marine arthropods, having been one of the most frequent fossil groups in the period between the Cambric and the Devonian, and are at this time still quite biologically diverse. Trilobite fossils are considered index fossils because they change slowly over time, exhibit specific characteristics and have lived in large areas of the world.

Index fossils refer to an organism that has had a wide geographical distribution in the past, although it has lived for short periods of time, and whose remains have not only been conserved in abundance but are also easy to identify.