Answer:
A. a fossil that can be used to establish the absolute age of a rock layer
Explanation:
An index fossil happens when a plant or animal is preserved in the rock record of the Earth. It needs to be characteristic of a particular span of time or environment. For it to be really useful, the index fossil should be easily recognizable, abundant over a large area, and be present for only a short time period. Index fossils are the basis for defining boundaries in the geologic time scale and for the correlation of strata.
