**Core Concept**
Sleep spindles are brief bursts of electrical activity in the brain, typically occurring during the stage of non-rapid eye movement (NREM) sleep. They are characterized by a specific pattern on an electroencephalogram (EEG) and play a crucial role in memory consolidation and protection from external stimuli.

**Why the Correct Answer is Right**
Sleep spindles are most prominent during Stage 2 NREM sleep, where they are part of the normal sleep cycle. This stage is characterized by the presence of K-complexes and sleep spindles, which help to distinguish it from other stages of sleep. The EEG during Stage 2 NREM sleep shows a more synchronized and slower activity compared to wakefulness, with the appearance of sleep spindles.

**Why Each Wrong Option is Incorrect**
**Option A:** This stage is characterized by rapid eye movements and low-voltage, mixed-frequency EEG, which is not typical for sleep spindles.
**Option B:** Stage 3 NREM sleep is characterized by slow-wave sleep (SWS), where delta waves are the prominent feature, not sleep spindles.
**Option D:** REM sleep is characterized by rapid eye movements, low-voltage, mixed-frequency EEG, and high brain activity, which is not conducive to the presence of sleep spindles.

**Clinical Pearl / High-Yield Fact**
Sleep spindles are a critical component of normal sleep architecture, and their absence or alteration can be indicative of various sleep disorders, such as insomnia or sleep apnea.

**Correct Answer: C. Stage 2 NREM sleep. This stage is characterized by the presence of K-complexes and sleep spindles.**