The answer is a diaphragm.
First of all, thorax, larynx, and lungs are not muscles. The diaphragm is a thick muscle in the lower part of the chest. It has a great role in breathing. It contracts and increases the thorax volume, thus, it inflates the lungs.
A thorax is the part of anatomy located between the neck and the abdomen. The lungs are the organs of respiration located inside the thorax. A larynx is also involved in breathing. It is located in the neck.