## Core Concept
The question tests understanding of electroencephalography (EEG) rhythms and their frequency ranges. EEG rhythms are classified based on their frequency: delta (δ), theta (θ), alpha (α), beta (β), and gamma (γ) waves. Each type of wave is associated with different states of brain activity.

## Why the Correct Answer is Right
The correct answer, delta (δ) waves, has the lowest frequency range among EEG rhythms, typically ranging from 0.5 to 4 Hz. Delta waves are most commonly observed during deep sleep, in infancy, and in serious pathologies. They are associated with unconsciousness and the release of hormones that help with deep sleep and restoration.

## Why Each Wrong Option is Incorrect
- **Option A: α (Alpha) waves** have a frequency range of 8-12 Hz, which is much higher than delta waves. They are typically seen in relaxed yet alert individuals.
- **Option B: θ (Theta) waves** have a frequency range of 4-7 Hz, which is higher than delta waves but lower than alpha waves. Theta waves are often observed during drowsiness or early sleep.
- **Option C: β (Beta) waves** have a frequency range of 13-30 Hz, which is significantly higher than delta waves. Beta waves are associated with active, anxious thinking and active concentration.

## Clinical Pearl / High-Yield Fact
A key point to remember is that delta waves are often considered abnormal when seen in awake adults, as they are typically associated with deep sleep or certain pathological conditions. The presence of delta waves in an awake adult EEG can indicate severe brain dysfunction.

## Correct Answer: D. δ (Delta).