Which of the following best describes a species that is termed "extinct"?

A species that is termed "extinct" best describes a situation where there are no known living individuals left. This status indicates that the species has completely disappeared from the planet, meaning that no members of this species remain alive in any habitat. Extinction can occur due to various factors such as environmental changes, loss of habitat, predation, disease, or competition from other species.

In contrast, the other choices refer to conditions or characteristics of species that are still existing. For example, rapidly reproducing species or those that have adapted to new environments are still alive and continuing their existence. Similarly, a newly discovered species would imply that there are living individuals of that species, possibly yet to be studied or classified in detail. Thus, the definition of extinction clearly identifies the absence of any living members as the key factor.