**Core Concept**
The electroencephalogram (EEG) is a tool used to measure electrical activity in the brain. Different states of consciousness and brain activity patterns can be distinguished based on the frequency and amplitude of brain waves. In particular, the EEG can help differentiate between states of relaxation and alertness.

**Why the Correct Answer is Right**
When a person is awake but relaxed with eyes closed, the EEG typically exhibits a dominant frequency of alpha waves (8-12 Hz). Alpha waves are a type of brain wave that corresponds to a state of relaxation, reduced cortical activity, and decreased sensory input processing. The presence of alpha waves indicates that the subject is not in a state of deep sleep or high alertness, but rather in a relaxed, closed-eyed state.

**Why Each Wrong Option is Incorrect**
* **Option A:** Beta waves are associated with active, engaged, or anxious thinking and active concentration. They are typically higher in frequency (13-30 Hz) than alpha waves and are not characteristic of a relaxed, closed-eyed state.
* **Option B:** Delta waves are slow waves (0.5-4 Hz) associated with deep sleep, unconsciousness, or coma. They are not typically seen in a relaxed, awake state.
* **Option C:** Theta waves are intermediate in frequency (4-8 Hz) and are often associated with drowsiness, sleep, or meditation. While theta waves may be present in a relaxed state, they are not the dominant frequency in this case.

**Clinical Pearl / High-Yield Fact**
In general, EEG findings can be used to differentiate between various states of consciousness, including wakefulness, sleep, and coma. Familiarity with the typical EEG patterns associated with different states is essential for accurate interpretation.

**Correct Answer: C.**