When is an animal considered no longer living?

An animal is considered no longer living upon permanent cessation of circulation because this reflects a fundamental biological definition of death. When circulation ceases permanently, it indicates that the heart has stopped functioning properly to deliver oxygen and nutrients to tissues, leading to a breakdown of cellular functions and eventually organ failure. This is a widely accepted clinical criterion for determining death in animals, as it signifies that the vital processes necessary to sustain life have irreversibly stopped.

The other options, while they may describe states that can occur in living animals, do not provide the clear and definitive boundary that permanent cessation of circulation does. An absence of response or inability to interact with the environment might be temporary conditions, such as during anesthesia or illness, where the organism could still potentially recover. Similarly, the notion of inactivity over a specified duration lacks a concrete physiological basis, as it does not necessarily correlate with the irreversible biological processes that define death.