Final answer:
Index fossils are the best type of fossils to use for determining the age of a rock layer.
Step-by-step explanation:
The BEST type of fossil to use when determining the age of a rock layer is an index fossil. Index fossils are fossils of organisms that lived for a relatively short period of time but were widespread geographically. They can be used to correlate and date rock layers found in different areas. For example, if an index fossil is found in a certain rock layer in one location and the same fossil is found in a rock layer in another location, it can be inferred that the rock layers in both locations are of similar age.
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