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

The answer is "d. all of the above".

Trilobites are index fossils because they "changed slowly through time , exhibit specific characteristics , lived over large areas of the world".

Trilobite fossils are discovered around the world, with a huge number of known species. Since they showed up rapidly in topographical time, and shed like different arthropods, trilobites fill in as astounding index fossils, empowering geologists to date the age of the stones in which they are found.