Answer : Corpus Callosum
Explanation : By the end of the preschool period, the brain structure called the Corpus Callosum becomes considerably thicker, with individual fibers that helps to co-ordinate brain functioning between the two hemispheres.
Corpus Callosum by definition is a thick band of nerve fibers that divides the cerebral cortex lobes into left and right hemispheres in the brain, which connects the left and right sides of the brain allowing for communication between both hemispheres.