Which of the following best describes an index fossil?

An index fossil is defined as an organism that existed for a relatively short, well-defined period of geologic time and is widely distributed in sedimentary rock layers. These fossils are particularly valuable for dating and correlating the age of the rocks in which they are found. Because they have a brief time range and a wide geographic distribution, geologists can use index fossils to identify specific time periods in the geologic record, allowing for effective dating of the rock layers across different locations.

This characteristic of having a limited lifespan makes these fossils effective indicators of a specific time in Earth's history, thus enabling scientists to establish relative ages of the rocks and the fossils found within them, which is central to the practice of geologic mapping.

In contrast, the other options do not accurately capture the principles surrounding index fossils: some may refer to the habitats or specific mineral contents, which do not directly pertain to the time limitation and wide distribution characteristics that define an index fossil.