Among the options listed, the correct answer is e) abdominal muscles.
During forced expiration, the abdominal muscles play a crucial role. They contract to increase abdominal pressure, which pushes the diaphragm upward and helps to expel air from the lungs more forcefully. While the diaphragm is primarily responsible for normal breathing (inhalation and exhalation), it is the abdominal muscles that are specifically engaged in activities requiring a more forceful exhalation, such as during exercise or coughing.
In contrast, the other muscles listed—external intercostals, sternocleidomastoid, and scalenes—are more involved in the processes of inhalation or normal breathing rather than forced expiration.