Final answer:
The rubrospinal and corticospinal tracts both terminate in the spinal cord, where they influence lower motor neurons in voluntary muscle movement control.
Step-by-step explanation:
The rubrospinal and corticospinal tracts are both descending pathways in the central nervous system that play a role in the control of voluntary movements. The rubrospinal tract originates from the red nucleus in the midbrain and terminates mainly in the spinal cord, where it influences motor neurons that control the distal musculature, particularly of the arm. Similarly, the corticospinal tract, which begins at the cerebral cortex, primarily terminates in the spinal cord, providing pathways for the brain to directly influence the activity of lower motor neurons. These lower motor neurons, which are the target of upper motor neurons like those in the rubrospinal and corticospinal tracts, innervate skeletal muscles. The answer to the question, therefore, is a) Spinal cord.