Final answer:
The absence of brain activity differentiates between clinical and biological death, as it indicates a total and irreversible end to cerebral functions and the capability to regain consciousness.
Step-by-step explanation:
The single physiological condition that differentiates between clinical and biological death is C) Absence of brain activity. Clinical death occurs when the heart stops beating and breathing ceases, which can sometimes be reversed through resuscitation. Biological death, which is irreversible, is characterized by the total lack of brain activity, including the absence of brainstem reflexes. This brain activity cessation is crucial because it signals the end of all cerebral functions and the loss of the capacity to regain consciousness, even if other bodily functions are artificially maintained.
Body temperature abnormalities, such as hyperthermia or hypothermia, can affect heart rate and the strength of contractions, and can lead to clinical death if not corrected. However, it is the cessation of brain activity that classifies a state as biological death, which implies an irreversible end to an individual's life. Blood pressure, body temperature, and the detection of a pulse are all clinical signs used to assess vital signs, but they alone do not define biological death.