Final answer:
The muscle that causes the earthworm to shorten and become thicker is the circular muscles.
Step-by-step explanation:
The muscle that causes the earthworm to shorten and become thicker is the circular muscles (option a).
Earthworms move by waves of muscular contractions of the skeletal muscle of the body wall hydrostatic skeleton, called peristalsis, which alternately shorten and lengthen the body. Shortening of the circular muscles draws the posterior portion of the body forward, making the earthworm thicker and shorter.