Final answer:
The best characteristic for an organism to be an index fossil is that it was commonly fossilized during a specific period, allowing for precise geological dating of the layers in which it is found. So the correct option is c.
Step-by-step explanation:
The characteristic of an extinct organism that makes it best suited to be used as an index fossil is c. an organism that was commonly fossilized during a specific period.
Index fossils are pivotal in determining the age of rock layers and correlating the geological age across different areas. These fossils are highly useful to paleontologists as they must be from species that were widespread, plentiful, and only existed for a short geologic period. This unique combination of attributes means that once these fossils are discovered in a particular geological layer, they can provide a relatively precise date for that layer. This makes research easier when correlating across the globe, as they were not limited to any particular environment (such as option b) nor continuously changing over a wide range of time (such as option d). Also, they ought to be recognizable and easily distinguishable from other species, which is not directly related to evolutionary change over time as stated in option a.