The correct answer is A. Make your lecture very simple and spare some extra time for him.
A mentally retarded student may have difficulty understanding and following lectures. It is important to make your lectures as simple as possible and to provide extra time for the student to ask questions and get help if needed. You can also try to make your lectures more interactive by asking questions and getting the student involved in discussions.
Option B is not a good option because it would be unfair to the other students in the class. The student should not be penalized for his disability.
Option C is also not a good option because it would be cruel to the student. The student should be given the opportunity to learn and succeed in school.
Option D is also not a good option because it would be unsupportive. The student needs help and support, not rejection.
It is important to remember that all students, regardless of their abilities, deserve the opportunity to learn and succeed. By making your lectures simple and providing extra time for the student, you can help him to succeed in your class.