Final answer:
Brain death occurs when a person no longer responds to external stimuli or has reflexes, yet may still breathe for a time. The answer is 'b. Shortly after brain death', indicating this condition typically arises soon after the brain stops functioning due to lack of oxygen.
Step-by-step explanation:
The condition described in the question where a person no longer has reflexes or response to vigorous external stimuli but may still be able to breathe and survive for some time after death is known as brain death. The most appropriate answer to complete the sentence is:
b. Shortly after brain death. This happens because although cardiac function may be present for a short period, the brain lacks oxygen, leading to irreversible damage. The affected individual does not respond to external stimuli and has lost reflexive functions. It's crucial in such cases to act promptly since brain cells start dying after approximately six minutes without oxygen. However, in some exceptional cases, such as cold-water drowning, the lack of oxygen can be tolerated for a longer period, particularly in children.