"during expiration, which best describes the role of the diaphragm? "

During respiration, the diaphragm moves upwards and returns to dome shape from the flattened shape before during expiration.

This movement of diaphragm can decrease the lung volume, increase the pressure inside and thus force the air out. This process is expiration.

However, if we breathe in again, which makes that inhalation, the diaphragm moves downwards into a flat shape so that it can increase the lung volume and thus we can breathe in as much air we can. This process is completely opposite from expiration.