Final answer:
Identical twins generally have similar gray and white matter volumes in their brains, due to their genetic identity, which comes from a single fertilized egg that splits into two.
Step-by-step explanation:
Brain scans reveal that identical twins typically have similar gray matter volume and similar white matter volume, which is the first option. Gray matter consists of neuronal cell bodies and is involved in processing information in the brain. It primarily makes up the brain's outer cortex. White matter, on the other hand, consists of myelinated nerve fibers (axons) that connect different areas of gray matter within the central nervous system.
Identical twins are genetically the same, as they come from a single fertilized egg that splits, leading to the development of two individuals with virtually identical genetic material. As a result, they usually have very similar brain structures, including the volumes of gray and white matter. This similarity in brain structure is often reflected in similar cognitive function and possibly behavioral traits.