An index fossil is:

a. a fossil found in a particular site.

b. a fossil used to categorize a stratigraphic layer.

c. the ideal specimen of that species that all examples must follow.

d. the type specimen of a species.

An index fossil is important and useful for dating and correlating the strata of which it belongs.

The index fossil have a narrow time range and a wide spatial distribution. An index fossil is used in the identification of related geologic formations.

Hence, (c) an index fossil is an ideal specimen of that species that all examples must follow.