B) Hair color
Hair color is something that is determined from parents' genes, and those genes don't affect how children learn. Other genes, however, may affect how children learn, how fast they develop, and if they have learning disabilities, so A cannot be it. Culture may affect how the children learns, such as if they are in an environment where learning is encouraged, so C cannot be it. A child's behavior may affect how they learn, such as if they learn by watching others or listening.