Down syndrome is caused by all except:
(A) Trisomy of 21 chromosome (B) Mosaicism of 21 chromosome (C) Robertsonian translocation of 21.21, 21.18 (D) Deletion of 21
(D) Three cytogenetic variants cause Down syndrome: 

Trisomy 21
Chromosomal translocation
Mosaicism

Trisomy 21 accounts for nearly 95% of all patients with Down syndrome.
It is now generally accepted that there are at least three forms of Down syndrome: 

One in which there is the typical trisomy 21 with 47 chromosomes (accounting for about 95% of cases)
Another termed the translocation type, in which there appear to be only 46 chromosomes, although the extra chromosome material of number 21 is translocated to another chromosome of G or D group, either 21/22 translocation or 21/21 translocation (about 3% of cases)
Another that is the result of chromosomal mosaicism (about 2%)

Rajendran R. Shafer's textbook of oral pathology. Edition 7. Page 729