An index fossil is:.a. a fossil found in a particular site. b. the ideal specimen of that species to which all later descriptions must refer. c. the type specimen of a species. d. a fossil used to categorize a stratigraphic layer.

Option d, a fossil used to categorize a stratigraphic layer

Explanation:

Fossils that are typical of a specific time range in the course of history of evolution of earth are generally termed as index fossil.  These fossils can be used to determine the age of the rock layers and fossils with in which they are found. An index fossil must have a unique identity so that it can be easily recognized.Along with this they must have a lived for a short span of time in horizontal rock layer which must be geographically widespread for matching up to huge distances.  

Hence, option D is correct