Which of the following is an essential characteristic of an index fossil?
the fossil organism lived only in a specific type of environment
the fossil organism lived for only a short span of geologic time
the fossil organism was exceptionally well preserved
the fossil organism didn't occur over a wide geographic area

the fossil organisms lived for only a short span of geologic time

Explanation:

The index fossils are very useful when it comes to determining the relative age of the rock layers, as well as the other fossils found in the same layer with them. These fossils have four basic characteristics, they appear only in limited geologic time, they are widespread, abundant, and distinctive. The fact that they are limited in their geologic existence is very good for the geologists as when they find a fossil like this they are immediately aware of the geologic period they are working with without having to make examinations. Also, these fossils can be found in abundance and in numerous places around the world, thus the same fossil can be used as a time scale almost everywhere. Being distinctive also helps a lot because the geologist can immediately be aware of the type of organism without having to further examine it.