**Core Concept**
The Glasgow Coma Scale (GCS) is a neurological scale used to assess the level of consciousness in a patient. It is a widely used tool in clinical settings to evaluate the severity of brain injury or illness.

**Why the Correct Answer is Right**
The GCS consists of three components: eye opening, verbal response, and motor response. A score of 9-12 is considered moderate, indicating a patient who is able to respond to verbal commands but may have some difficulty. This score corresponds to a patient who can open their eyes to verbal command, has a confused or disoriented verbal response, and has a flexion response to pain.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because a score of 3-8 is considered severe, indicating a patient who is unresponsive to verbal commands.
* **Option B:** This option is incorrect because a score of 13-15 is considered good, indicating a patient who is fully awake and alert.
* **Option C:** This option is incorrect because it does not specify the correct score range for moderate severity.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the GCS is a sensitive tool for assessing the severity of brain injury, and a moderate score indicates a need for close monitoring and possible intervention.

**Correct Answer: B. 13-15. Good (fully awake and alert)**