An environmental factor is a factor from your environment that influences you. (for example: Social influences, home life, etc.)
However, influences such as: Genetics, physical appearance etc, are not environmental factors because they are not influences from your background.
Therefore, since the question is calling for a factor that is NOT environmental, I would say the answer is “Difference in brain activity”
The other answers are not environmental because they are not an influence due to how you were raised or lived in your life.
Apologies if this is incorrect.