The correct answer is A) External intercostals.
The primary muscle responsible for inspiration is the diaphragm. However, when considering accessory muscles, the external intercostals play a significant role during the inhalation process. When these muscles contract, they lift the ribs and expand the thoracic cavity, which helps in drawing air into the lungs.
On the other hand, the internal intercostals are primarily involved in expiration rather than inspiration. They assist in pulling the ribs down and inward during forced breathing, which is not their main function during quiet or normal breathing.
Thus, during normal inhalation, while the diaphragm is the main muscle, the external intercostals assist as key muscles that effectively increase the volume of the thoracic cavity, aiding in ventilation.