The thick muscle in the lower part of your chest is your
diaphragm
thorax
larynx
lungs

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.