Which of the following best describes a fossil?

A fossil is best described as the preserved remains or traces of ancient organisms, typically found in sedimentary rock. The correct choice, a dinosaur bone, exemplifies a specific type of fossil, as it consists of the mineralized remains of an extinct organism. Fossils provide critical evidence about the types of life that existed in the past and their evolutionary history.

The other options do not encapsulate the definition of a fossil adequately. A living organism is not a fossil; it is an organism that is currently alive. Ancient plant life could be referential to fossils, but it is too broad and does not specify that these remains are preserved. A modern artifact, such as something made by humans, is not related to biological remains but rather pertains to cultural history and craftsmanship. Thus, a dinosaur bone directly aligns with the definition and characteristics of a fossil.