The correct option is A diaphragm
Abdominal breathing takes place mostly with the help of the diaphragm. The diaphragm is a dome-shaped muscle that separates the chest cavity and the abdominal cavity. During inspiration the diaphragm contracts and the volume of the thoracic cavity increases. As the volume of the thoracic cavity increases, the pressure inside the lungs decreases. Since the pressure of air is lower within the lung than the atmosphere, air enters the lungs. During expiration, the diaphragm relaxes because of which the volume inside the lungs decreases. As the volume decreases the pressure inside the lungs increases and air moves out to the atmosphere.
Thoracic breathing mainly takes place with the help of external intercostal muscles.
During heavy breathing while exhaling, the relaxation of the diaphragm and the external intercostal muscles is not sufficient. So, other muscles such as the internal intercostal muscles also contract.
Sternocleidomastoid helps along with the diaphragm and the external intercostal muscles during inspiration while heavy breathing.