Answer:
b. primary motor cortex
Step-by-step explanation:
The white matter motor pathway is the the corticospinal pathway. This pathway begins at cerebral cortex and ends on interneurons present in spinal cord and lower motor neurons. The pathway controls the limb and trunk movement in the body. There are around one million myelinated neurons in corticospinal track.
The tract also includes premotor areas and primary somatosensory cortex. The neurons mostly begin in primary motor cortex area of the brain. Thus, option B is correct.