How is an "endangered species" defined?

An "endangered species" is defined as a species at risk of extinction. This classification indicates that the species faces significant threats that can lead to a declining population, habitat loss, or other factors that jeopardize its survival. Recognizing a species as endangered is crucial because it triggers conservation efforts and legal protections aimed at preventing its extinction.

In contrast, the other options do not accurately represent the definition of an endangered species. A stable population does not imply any risk of extinction and instead refers to a healthy and sustainable population level. A thriving species is one that is flourishing within its habitat, contributing to ecosystem balance. Lastly, a species that is temporarily vulnerable may face short-term challenges but does not necessarily face the imminent threat of extinction, which is the key aspect of being classified as endangered. Thus, only the designation of "at risk of extinction" directly aligns with the definition of an endangered species.