This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Unknown
Which of the following is an example of intellectual disability? 
Select and explain. 
A. Dyslexia, B. ADHD, C. Autistic spectrum disorder, D. Mental retardation
D: Mental retardation
Answer: D: Mental retardation 
Explanation: Ref. Oxford textbook of Psychiatry. Page. 488