**Core Concept**
Sleep spindles and K-complexes are characteristic EEG features of Stage 2 non-rapid eye movement (NREM) sleep, representing a period of light sleep with reduced cortical activity. This stage is crucial for memory consolidation and learning.

**Why the Correct Answer is Right**
During Stage 2 NREM sleep, the brain exhibits increased synchronization of neural activity, leading to the formation of sleep spindles and K-complexes. Sleep spindles are bursts of rapid, rhythmic activity (12-16 Hz) that last for a few seconds, whereas K-complexes are brief, high-amplitude waves followed by a burst of spindling activity. These EEG features help differentiate Stage 2 from other stages of sleep.

**Why Each Wrong Option is Incorrect**
**Option A:** This stage is incorrect because Stage 1 NREM sleep is characterized by a gradual decrease in consciousness and increased theta wave activity (4-8 Hz), but not by the presence of sleep spindles and K-complexes.
**Option B:** REM sleep is incorrect because it is characterized by rapid eye movements, increased brain activity, and low-amplitude, high-frequency EEG activity, which is distinct from the sleep spindles and K-complexes seen in Stage 2 NREM sleep.
**Option C:** Stage 3 NREM sleep, also known as slow-wave sleep, is incorrect because it is characterized by high-amplitude delta waves (0.5-4 Hz), which are not the same as the sleep spindles and K-complexes seen in Stage 2.

**Clinical Pearl / High-Yield Fact**
Remember the "A-B-C-D" stages of sleep: Stage 1 (A) is light sleep, Stage 2 (B) is characterized by sleep spindles and K-complexes, Stage 3 (C) is slow-wave sleep, and Stage 4 (D) is not a standard stage of sleep (the American Academy of Sleep Medicine has replaced Stage 4 with Stage 3, which now includes both slow-wave sleep and REM sleep).

**Correct Answer:** B. Stage 2 NREM sleep.