Eight-year-old ella can use scissors to cut small paper dolls out of construction paper, something she could not do at age three. what best accounts for her improving dexterity?

Increased myelination of the central nervous system. Dexterity with the hands is really just another way of looking at fine motor skill control. As humans age from infant to adult, their brain and it's control of their body is still growing, still developing. Neural connections are constantly being built in the early years of childhood development. Thus a baby who can only hold things with a closed fist moves on to a toddler who can hold things on their open palm or with a closed or only half-closed fist. As the child's brain continues to develop, their fine motor skills grow to. This is why they can hold things in their hands as tools at a later age than when they were a toddler - their brain is capable of communicating to their fine muscle motors much more to allow them to control what's in their hands.