The preserved remains or traces of ancient life.

Here's why this is the best description:

* Preserved Remains: This encompasses actual parts of organisms like bones, teeth, shells, or even soft tissues (though these are much rarer).

* Traces: This includes evidence of life's activities, like footprints, burrows, nests, or fossilized feces (coprolites).

* Ancient Life: Fossils are by definition from organisms that lived in the past, often millions or billions of years ago.

While other options like "hardened remains" or "evidence of past life" are partially correct, the most comprehensive and accurate description is "the preserved remains or traces of ancient life."