Answer: A) Serotonin
Explanation: Dopamine and Serotonin have been highly studied in the context of Autism Spectrum Disorder (ASD), however, a higher number of research studies suggest that abnormalities in serotonin levels or functioning in the brain may be associated with certain cases of ASD.
ASD is a complex disorder and is not solely attributed to any single neurotransmitter or factor. Multiple biological and environmental factors contribute to the development of autism.