Many older civilizations believed that dreams were metaphysical experiences wherein the soul was transported from the body to a completely different realm or spiritual plane.

Physiological theories claim that dreams are a product of neural firing in the brain, which can sometimes, according to some theorists, aid in the process of learning. This is why it is recommended to get good sleep before a test!

Psychological theories claim dreams are a way of processing issues in our lives, founded in Freud's initial dream theory, which posited dreams as ways we process repressed feelings and desires.

There are many who believe dreams are a combination of both theories; they are random, but still serve some purpose.

Stage 4 sleep is referred to as delta sleep, or deep sleep. It is very hard to wake someone in this stage of sleep. Unlike REM sleep, there are no eye or muscle movements. This is the time during which young children may wet their beds and/or experience night terrors.  

Sleep apnea is characterized by pauses or lack of breathing when sleeping. A lack of oxygen to the brain causes a buildup of carbon dioxide in the bloodstream. Accumulated carbon dioxide causes the person to wake up, gasping for air. Normal breathing upon wakefulness will allow normal oxygen flow until the person falls back asleep. Sleep apnea is more common among middle aged and overweight individuals. While genetics play a significant role, those with enlarged tonsils, allergies (which may cause swelling of the throat), sinus problems, or enlarged necks are also at risk of sleep apnea.

Memory is encoded during REM sleep. In the hippocampus, neurons fire four times as opposed to once as a result of ponto-geniculo-occipital (PGO) spikes. Memories from a day's activities and learning are encoded in the hippocampus four times, contributing to long-term memory. Theta waves are also associated with this stage of sleep and contribute to REM's restorative nature.  

REM sleep (or Rapid Eye Movement) is the stage in which dreams occur, because even though the body is essentially paralyzed, this stage is accompanied by intense activity in the brain. 

The term "healthy lifestyle" may be broadly defined from culture to culture. While sleep is healthy and good for the body, too much sleep may also be detrimental, or a sign of severe depression. This answer choice is too broad, and not specific enough in its limitations on sleep, to be accurate.

All the other answers are well known theories about the function of sleep.

Out of the 4 stages of sleep, NREM-1, NREM-2, NREM-3, and REM, the third stage is where the brain will emit delta waves. These waves are large and slow, as such, this stage of sleep is also known as slow-wave sleep. It is during this stage where it may be difficult to wake someone up.

Dreaming has been observed during REM sleep. Brain waves at this stage resemble the brain waves of an individual who is awake. The person's heart rate will increase as well as his or her breathing. Every few seconds, the individual's eyes may be observed darting from side to side. This movement signifies the beginning of a dream.

While the recuperation explanation may seem like a good answer, it cannot be considered a theory on its own.

"Sleep protects" is a theory that a species will adjust its sleep pattern in a manner where it is optimal given that species niche, or environmental context. For instance, if an animal has a limited ability to hide from its predators, it may sleep less.