What are fossils?

Fossils are specifically defined as the remains or traces of ancient organisms that have been preserved in rock over geological time. This can include the actual remains of the organisms, such as bones or shells, as well as traces such as footprints, burrows, or even impressions of soft tissues. The processes of sedimentation and mineralization allow these organic materials to transform into fossils, providing us with vital information about the history of life on Earth, ancient ecosystems, and how organisms evolved.

The other choices do not accurately describe fossils; ancient tools pertain to human artifacts, structures used for navigation are unrelated to organic remains, and natural formations like mountains and rivers refer to geological features rather than biological remains. Therefore, the correct definition centers on the preservation of biological evidence in geological contexts, which is captured in the choice that outlines fossils as traces or remains of ancient organisms.