Final answer:
The trachealis muscle is located between the esophagus and the trachea, as part of the fibroelastic membrane of the trachea itself.
Step-by-step explanation:
The trachealis muscle is found in the trachea, which is the windpipe that extends from the larynx toward the lungs. Specifically, the muscle is part of the fibroelastic membrane that closes the posterior surface of the trachea, connecting the C-shaped cartilages. This muscular membrane allows the trachea to stretch and contract slightly during breathing while preventing collapse. The muscle itself can contract to force air through the trachea during exhalation.
The correct answer to the question is D. between the esophagus and the trachea, as the trachealis muscle is located posterior to the C-shaped cartilages of the trachea and anterior to the esophagus.