What qualifies a fossil as an index fossil?

An index fossil is characterized by its wide geographical distribution and relatively short temporal range, which means it existed for a limited time in geological history. This makes option B the correct choice. Index fossils serve as important indicators for the relative dating of rock layers and help geologists correlate the age of different geological formations across vast distances.

The key aspects of an index fossil are its ability to indicate the age of the rock layers in which it is found and its widespread presence in various locations during its time of existence, allowing scientists to draw connections between different regions. This makes them invaluable in the study of Earth's history.

In contrast, fossils that are found in a limited area and existed for a long time do not provide significant insights into the specific time frame, as they could span multiple geological periods. Likewise, fossils that are common throughout all geological time would not serve as good markers for specific time intervals as they lack the necessary temporal constraint. Finally, fossils confined to just one geological layer do not help in correlating ages across different locations.