Answer:
D) Used as reference points in absolute dating
Step-by-step explanation:
Strictly speaking, the term index fossil is used in stratigraphy to designate fossils from taxonomic groups (usually of the genus or species category) on the basis of which biozones are defined, for relative dating of geological strata. In other words, these fossils are used as reference points in absolute dating.
In a broader, informal way, the term index fossil (or old fossil, characteristic fossil or stratigraphic fossil) is often used to generically designate the fossils of taxonomic groups that allow for finer, more detailed relative dates, or to identify specific biostratigraphic interval.