How Would Neuroscientists Explain the Early Adulthood Shifts in Behavior

How would neuroscientists explain the early adulthood shifts in behavior that are often seen in the early twenties?

A) There is a thickening of the corpus callosum, so information can travel more readily across the cerebral hemispheres.
B) There is increased myelination in the structures of the hindbrain-the pons, medulla, and cerebellum-which leads to greater control of impulsive behaviors.
C) There is a decrease in cortical activity in the diencephalon-the hypothalamus and thalamus-which in turn leads to a stabilizing of endocrine activities.
D) There is further development in the prefrontal cortex, which is involved in higher level thinking processes.