Final answer:
Rigor mortis was considered a clear sign of death in the past, as it visibly indicates the end of life without requiring advanced technology.
Step-by-step explanation:
Regarding the sign of death, in the past, without advanced medical equipment, rigor mortis was considered a clear sign. Rigor mortis, which refers to the stiffening of the joints and muscles of a body a few hours after death, is a postmortem change that signals the cessation of life. It was observable and did not require technology to confirm.
In contrast, some other options like a flat EEG or lack of higher cortical brain function were not widely used or not applicable in the past due to the lack of necessary medical technology or clear understanding. The option mentioning 'moderate brain stem function' would not be a sign of death as any brain stem activity is typically indicative of some level of life function.