What does extinction indicate about a species?

Extinction specifically refers to the point at which there are no individuals of a species remaining alive. This means that the species can no longer reproduce or contribute to the ecosystem. Once a species is declared extinct, it signifies that all living members have died out, often due to factors such as habitat loss, environmental changes, or human activities. Therefore, the correct answer highlights the definitive conclusion of a species' existence, marking an end to its lineage on Earth.

The other options, while related to population dynamics and biological processes, do not accurately reflect the meaning of extinction. For instance, a declining population does not necessarily mean the species is extinct; it could still be on the verge of extinction but not yet completely wiped out. Adaptation and evolution also imply the continuation of a species, which contradicts the very concept of extinction.