Which of the following is the most important muscle in the normal respiratory process of an adult?

A. Trapezius: The trapezius is a muscle of the upper back and neck and does not play a primary role in normal respiration. B. Diaphragm: The diaphragm is the most important muscle for respiration. It contracts and moves downward during inhalation, increasing the thoracic cavity's volume and drawing air into the lungs. C. Rectus abdominus: The rectus abdominus is a core muscle that helps in forced expiration but is not essential in normal breathing. D. Sternocleidomastoid: This muscle assists with forced inspiration, but it is not the primary muscle for regular breathing.