Final answer:
The Transverse abdominis is the muscle that primarily functions to compress the abdomen, making it the correct answer to the student's question.
Step-by-step explanation:
Of the muscles listed, d. Transverse abdominis is the one that primarily functions to compress the abdomen. The transverse abdominis is the deepest of the flat muscles of the abdomen, wrapping around the torso and performing the compression of the abdominal organs. Its horizontal fiber orientation makes it uniquely suited for this purpose. On the other hand, the rectus abdominis flexes the spine and is known for its 'six-pack' appearance thanks to a long tendon called the linea alba, not primarily for compression. The diaphragm is essential for respiration, as it moves downward when contracting to allow the lungs to expand. Lastly, the internal intercostal muscles help in forced exhalation by drawing the ribs closer together, but they don't compress the abdomen.