An index fossil is usually defined as those fossil that appeared for a short geological time and were widely distributed over the surface of the earth. These fossils are extremely rare, and it plays an important role in determining the age of a rock, and it also helps in the correlation of rocks. A noticeable number of index fossil species were observed in different places on earth which were deposited on the rock sequences in the geological time.
It enables a geologist to categorize the different stratigraphic layers.
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.
The phenomenon is called an equinox. More specifically when it is September it is called the autumnal equinox ,and when it is March it is called the vernal equinox. Hope this helped!
