Final answer:
Decapitation (option A) is the definitive sign of death among the options given as it results in irreversible brain-body separation. In microbiology, 'thermal death time' describes the time needed to kill all microbes in a sample at a set temperature.
Step-by-step explanation:
Sign of Definitive Death and Microbial Death Time
The question posed relates to signs indicative of death in an individual. Of the options presented, decapitation is a definitive sign of death. This is because decapitation results in an irreversible separation of the brain from the body, leading to the cessation of all vital functions.
While pulselessness, apnea, and unresponsiveness might be concerning signs that are often associated with severe medical conditions or states that can lead to death, they are not definitive on their own since resuscitation and medical intervention might reverse these conditions.
In the context of microbiology, the term that describes the time required to kill all microbes within a sample at a given temperature is referred to as the thermal death time. This term is chosen over other options such as D-value, thermal death point, or decimal reduction time as it specifically addresses the duration necessary to eliminate a microbial population at a specific temperature.