Final answer:
Galen is the scientist who proposed that the brain controls muscle motions through the nervous system.
Step-by-step explanation:
The scientist who proposed that the brain controls the motions of the muscles through the nervous system is D) Galen. Galen's work in medical science was foundational in the field, showcasing an early understanding of the human body's functions. His theories on the nervous system controlling muscle motion were advanced for the time and stand as a significant accomplishment in ancient medicine.