Final answer:
During the last trimester, the main job of the fetus is brain development, which involves the myelination of the axons of the spinal cord for efficient nervous system functioning.
Step-by-step explanation:
The main job of the fetus during the last trimester is brain development. During this period, the axons of the spinal cord begin to be myelinated, which means they are coated in electrically insulating glial cell sheaths. This myelination process is important for efficient nervous system functioning.