**Core Concept**
The question is testing the ability to identify the characteristic EEG pattern associated with a specific brain state in adults. The underlying principle is the distinction between different stages of wakefulness and sleep, as reflected in the electroencephalogram (EEG) readings.

**Why the Correct Answer is Right**
When an adult is awake but mind-wandering with eyes closed, the brain is in a state of relaxed wakefulness. This state is characterized by a specific EEG pattern known as **theta waves**. Theta waves are a type of brain wave with a frequency of 4-8 Hz, which is slower than the beta waves seen during focused attention. This frequency range is typical of the brain's default mode network, which is active when we are not actively engaged in tasks and are allowing our minds to wander.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, so we'll move on to the remaining choices.
* **Option B:** This could potentially be a distractor, but without knowing the specific EEG pattern associated with it, we can't be sure. However, we can say that it's unlikely to be the correct answer since it's not mentioned in the context of mind-wandering and eyes closed.
* **Option C:** This is not a valid EEG frequency range. While alpha waves (8-12 Hz) are typically seen in relaxed, closed-eye conditions, they are not the same as theta waves.

**Clinical Pearl / High-Yield Fact**
In clinical practice, EEG patterns can be used to diagnose and monitor various neurological conditions. Familiarity with different EEG patterns, including theta waves, can help healthcare professionals better understand the underlying brain activity and make informed decisions about patient care.

**Correct Answer: B. Theta waves.**