The following is an example of wakefulness: a. Jemma, competing in a marathon b. Mercy, unconscious after being hit on the head c. Petro, immediately before entering REM sleep d. Samuel, sleepwalking in the middle of the night

A, competing in a marathon

Explanation:

Wakefulness is a brain state, a conscious state where an individual is able to think in a cognitive manner and responds to the external world.

The opposite of wakefulness is being asleep, where the external input to the brain is not being processed.

Jemma, competing in a marathon is perfectly aware of her actions and her thinking is clear. In all the other examples, the people are unconscious, or in a sleep or a sleep-alike state (sleepwalking), where their brain is not involved in making decisions or processing information.