**Core Concept**
REM (Rapid Eye Movement) sleep is characterized by low muscle tone, increased brain activity, and vivid dreams. This stage is crucial for memory consolidation and learning.

**Why the Correct Answer is Right**
REM sleep occurs in cycles throughout the night, with each cycle lasting around 90-120 minutes. During this stage, the brain's neurotransmitters, such as norepinephrine, serotonin, and acetylcholine, are deactivated, leading to a decrease in muscle tone and a decrease in the ability to respond to external stimuli. The brain's default mode network is also active during REM sleep, which is responsible for the vivid dreams and increased brain activity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because REM sleep is not characterized by high muscle tone. In fact, muscle tone is decreased during this stage.
**Option B:** This option is incorrect because REM sleep is not the stage where the brain's neurotransmitters are most active. Instead, the brain's neurotransmitters are deactivated during this stage.
**Option C:** This option is incorrect because REM sleep is not the stage where the brain's default mode network is least active. Instead, the default mode network is active during REM sleep.

**Clinical Pearl / High-Yield Fact**
REM sleep is essential for memory consolidation, and deprivation of REM sleep can lead to impaired learning and memory.

**Correct Answer: D. REM (Rapid Eye Movement) sleep.**