The relaxation of external intercostal muscles and diaphragm leads to return of diaphragm, ribs, and sternum to resting position thus restoring thoracic cavity to pre-inspiratory volume. The pressure in the lungs increases and the air is exhaled. So, the correct answer is option A.
During inspiration, the diaphragm contracts and pulls downward while the muscles between the ribs contract and pull upward. This increases the size of the thoracic cavity and decreases the pressure inside. As a result, air rushes in and fills the lungs.
During expiration, the diaphragm relaxes, and the volume of the thoracic cavity decreases, while the pressure within it increases. As a result, the lungs contract and air is forced out.