Answer:
C. The area where the fossils were found was once underwater.
Explanation:
The conclusion that best explains the discovery of marine fossils at the top of a mountain is:
C. The area where the fossils were found was once underwater.
This conclusion is supported by the understanding that geological processes, such as plate tectonics, can cause the uplift of oceanic sediments and marine organisms, forming mountains. This process can lead to the presence of marine fossils in what are now elevated regions.