**Core Concept**
The retromylohoid area is a region of the anterior abdominal wall that contains several muscles, including the external oblique, internal oblique, and transversus abdominis. The muscle involved in this area is responsible for contributing to the thickness of the abdominal wall.

**Why the Correct Answer is Right**
The correct answer is the external oblique muscle. During the recording of the retromylohoid area, the external oblique muscle is the primary contributor to the thickness measured. This is because the external oblique muscle is the most superficial of the three abdominal muscles in the retromylohoid area and has the largest cross-sectional area. The external oblique muscle is innervated by the lower six thoracic nerves and is responsible for movements such as rotation, flexion, and lateral flexion of the vertebral column.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because the internal oblique muscle is deeper than the external oblique muscle and does not contribute as much to the thickness of the retromylohoid area.
* **Option B:** Incorrect because the transversus abdominis muscle is the deepest of the three abdominal muscles and is not the primary contributor to the thickness measured in the retromylohoid area.
* **Option D:** Incorrect because the rectus abdominis muscle is not located in the retromylohoid area and is not involved in the recording of this region.

**Clinical Pearl / High-Yield Fact**
Remember that the external oblique muscle is the primary contributor to the thickness of the abdominal wall in the retromylohoid area, and its measurement is often used as an indicator of abdominal muscle mass and strength.

**Correct Answer:** C.