Final answer:
The hallmark of frailty in an aged patient is significantly reduced reserve to withstand physiologic stress.
Step-by-step explanation:
The hallmark of frailty in an aged patient is B. The patient's reserve to withstand physiologic stress is significantly reduced. Frailty is a state of vulnerability due to age-related declines in various physiological systems. It is characterized by decreased physiological reserve, increased vulnerability to stress, and a higher risk of adverse health outcomes.