Final answer:
An index fossil is a recognizable, abundant fossil that existed for a short period of time and had a wide geographic distribution. The fossilized shell that looks like a clam would make a great index fossil for these three rock outcrops because it meets the criteria of an index fossil.
Step-by-step explanation:
An index fossil is a fossil that is easily recognizable, abundant, existed for a short period of time, and had a wide geographic distribution. These fossils can be used to help determine the relative age of rock outcrops. In this case, the fossilized shell that looks like a clam would make a great index fossil for the three rock outcrops because it meets the criteria of an index fossil.
By finding this fossilized shell in the rock outcrops, we can infer that the rocks are of a similar age and were deposited during the same time period when this shell species was abundant. This shell would be easily recognizable and have a wide distribution, allowing for correlation of the rock outcrops.
Furthermore, the fact that the shell is fossilized indicates that the rock outcrops were once underwater, as the fossilization process occurs in sedimentary environments where organic material gets buried and replaced by minerals over time.