Concept:The diaphragm is the dome-shaped muscular sheet separating the thoracic cavity from the abdominal cavity.Explanation:The thoracic cavity contains the heart and lungs, while the abdominal cavity contains the digestive organs.The diaphragm contracts and flattens during breathing, helping air move in and out of the lungs.Among the listed options, the diaphragm is the muscle that separates these two cavities.Answer:A. diaphragm