Which of the following is primarily responsible for increasing the volume of the thoracic cavity?

The diaphragm is primarily responsible for increasing the volume of the thoracic cavity during the process of inhalation. It is a dome-shaped muscle located at the base of the thoracic cavity. When the diaphragm contracts, it moves downward, which increases the vertical dimension of the thoracic cavity. This downward movement creates a negative pressure in the pleural cavity relative to atmospheric pressure, allowing air to flow into the lungs.

The intercostal muscles complement the diaphragm during breathing by facilitating the elevation and expansion of the rib cage, but their function supports rather than initiates the primary change in volume achieved by the diaphragm. Meanwhile, abdominal muscles are more active during forced expiration rather than during the increase of thoracic volume during inhalation. The scalene muscles also assist in elevating the first two ribs, contributing somewhat to thoracic expansion but not to the degree that the diaphragm does.

Thus, the diaphragm is the key muscle that significantly increases the volume of the thoracic cavity, making it the correct answer.