**Core Concept**
The prevertebral space refers to the area in the neck that lies anterior to the vertebral bodies and posterior to the esophagus, trachea, and thyroid gland. The thickness of this space is an important radiological parameter used to assess for potential infections or other pathologies.

**Why the Correct Answer is Right**
The correct answer is based on the normal thickness of the prevertebral soft tissues as measured on a lateral neck radiograph. The normal thickness of the prevertebral space in adults is generally considered to be less than 7 mm at the level of C6. This measurement is essential for diagnosing conditions such as retropharyngeal abscess or hematoma, where the prevertebral space may become enlarged.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too low, and a prevertebral space thickness of less than 5 mm is typically considered normal at the level of C6.
**Option B:** This option is incorrect because it is too high, and a prevertebral space thickness of greater than 10 mm is typically considered abnormal at the level of C6.
**Option C:** This option is incorrect because it does not provide a specific measurement, making it an ambiguous choice.

**Clinical Pearl / High-Yield Fact**
When interpreting lateral neck radiographs, it's essential to remember that the prevertebral space thickness can vary with the level of the neck, and a thickness of less than 7 mm is generally considered normal at the level of C6.

**Correct Answer:** C. 7 mm