Final answer:
Contraction of the trachealis muscle is the correct answer, as it can contract to reduce the trachea's diameter during exhalation, leading to an increase in the velocity of airflow through the trachea.
Step-by-step explanation:
The correct answer associated with an increase in the velocity of airflow through the trachea would be C. Contraction of the trachealis.
The trachealis muscle is a band of smooth muscle that connects the open parts of the C-shaped tracheal cartilages at the back of the trachea. During exhalation, the trachealis muscle can contract, reducing the diameter of the trachea. This contraction increases the speed of airflow as air is forced out of the lungs, essentially functioning in a manner similar to pursing your lips to blow out air more forcefully.
Option A, the relaxation of the cricoid cartilage, would not increase airflow velocity. Similarly, options B, contraction of the esophagus, and D, contraction of the carina, are not directly related to changes in airflow through the trachea. The esophagus is not involved in air transport, and the carina is the point at which the trachea divides into the two primary bronchi and does not directly influence tracheal airflow velocity.