Final answer:
An index fossil is a fossil from a very short-lived, geographically widespread species known to exist during a specific interval of geologic time. Index fossils are easily recognizable, abundant, existed for a short period of time, and had a wide geographic distribution, making them useful for determining the age of rocks.
Step-by-step explanation:
An index fossil is a fossil from a very short-lived, geographically widespread species known to exist during a specific interval of geologic time. It is true that index fossils are easily recognizable, abundant, existed for a short period of time, and had a wide geographic distribution. When you find an index fossil in a rock, it can help determine the approximate age of the rock.