Final answer:
The deepest stage of sleep is during Stage III, referred to as deep or slow-wave sleep characterized by delta waves. This is distinct from Stage 2, which features sleep spindles and K-complexes associated with learning and memory. The correct option is A.
Step-by-step explanation:
The deepest stage of sleep is experienced during Stage III of non-REM sleep. This stage is often referred to as deep sleep or slow-wave sleep and is characterized by low frequency, high amplitude delta waves. During Stage III, an individual's heart rate and respiration slow down significantly, making it much more difficult to awaken the sleeper.
Sleep spindles and K-complexes, which are thought to play a role in learning and memory, are most often associated with Stage 2 sleep. These sleep spindles are a rapid burst of high frequency brain waves, serving as one of the defining characteristics of this particular stage. It is during Stage 2 that the body goes into a state of deep relaxation.