For air to enter the lungs (process of ventilation), the intrapulmonary pressure must be less than atmospheric pressure so air can be pulled inward. Select the movement of respiratory muscles that makes this happen during inspiration.

At the time of inspiration, contraction of diaphragm takes place that enhances the chest cavity.

Explanation:

One of the prime muscle that takes part in the process of respiration is the diaphragm, which is situated below the lungs. It is a dome-shaped and large muscle, which spontaneously goes through a contraction, and largely the process takes place involuntarily.  

The contraction of diaphragm takes place at the time of inhalation, which flattens and increases the chest cavity. Due to the contraction, a vacuum is generated that makes the air to get in within the lungs. In contrast, the diaphragm relaxes at the time of exhalation and moves back to its original shape, thus, making the air move out of the lungs.