Final answer:
The increase in attention span from first graders to fourth graders is primarily due to the myelination of neurons, which enhances the efficiency of neural communication.
Step-by-step explanation:
The question asks which biological change is primarily responsible for the increase in attention span from first graders to fourth graders. The correct answer is a) Myelination of neurons. Myelination refers to the development of a fatty sheath around the axons of neurons, which speeds up the transmission of electrical impulses and increases the efficiency of neural communication. This process helps to improve cognitive functions such as attention span. While other options like synaptic pruning, hormonal changes, and brain lateralization also contribute to brain development, they are not the primary reasons for improved attention span at the stage between being a first grader and a fourth grader.