Which of the following genetic disorder can cause mental retardation?

The correct option is C Down's syndrome
Down's syndrome is a genetic disorder due to the presence of an extra copy of a chromosome that leads to the abnormal growth and mental retardation. Down's syndrome generally is due to trisomy 21, i.e., presence of chromosome 21 in three copies. The affected individuals are mentally retarded, short in stature and have a broad, short skull, small mouth.

Thus, the correct answer is option C.