Final answer:
The correct role of the diaphragm in infant breathing is that it contracts during inspiration, which allows air to flow into the lungs as the thoracic cavity expands.
Step-by-step explanation:
The role of the diaphragm during normal breathing in infants, as well as in adults, is crucial for the process of pulmonary ventilation which includes both inspiration and expiration. During inspiration, the diaphragm contracts and moves downward, increasing the space in the thoracic cavity and allowing the lungs to expand. This expansion creates a partial vacuum, causing air to flow in.
On the contrary, during expiration, the diaphragm relaxes and moves upwards, reducing the space in the thoracic cavity and causing the lungs to compress and push air out. To answer the student's question, option A is correct: the diaphragm contracts during inspiration.