Final answer:
Improved motor coordination in early childhood is most directly contributed by the myelination of the corpus callosum, which enhances neural transmission and interhemispheric communication. The answer is option B.
Step-by-step explanation:
The most direct contributor to improved motor coordination in early childhood is myelination of the corpus callosum. This structure connects the two hemispheres of the brain and myelination helps speed up neural transmission, thus facilitating better coordination and communication between the hemispheres.
This process allows for smoother and more coordinated motor functions. The cerebellum is also critical as it plays a role in procedural learning, involved in motor skills like riding a bike or throwing a football by acting as a comparator for voluntary movement.