When the diaphragm contracts, the vertical dimensions of the thoracic cavity increase. So, the correct answer is a) relaxes increase.
Here’s why: The diaphragm is a dome-shaped muscle that separates the thoracic and abdominal cavities. When it contracts, it flattens out, moving downward and increasing the space in the thoracic cavity. This decrease in pressure within the thoracic cavity allows air to flow into the lungs, facilitating breathing. Simultaneously, the abdominal cavity gets compressed as the diaphragm descends, pushing down the abdominal organs. Thus, the contraction of the diaphragm essentially draws air into the lungs by expanding the thoracic cavity.