Final answer:
The contraction of the external intercostal muscles increases the volume of the thoracic cavity during inspiration by raising the rib cage.
Step-by-step explanation:
The contraction of the external intercostal muscles will increase the thoracic cavity volume during inspiration. These muscles are responsible for expanding the rib cage when they contract.
As the rib cage moves upward and outward, the volume of the thoracic cavity increases, leading to a decrease in pressure inside the pleural cavities, which in turn causes air to flow into the lungs. By contrast, the internal intercostal muscles are involved in expiration, not inspiration, as they draw the ribs together and reduce the volume of the thoracic cavity.
Therefore, the correct answer is c. the external intercostals.