Trisomy of the 21st pair of chromosomes as a result of autosomal primary nondisjunction is the cause of Down's syndrome. Langdon Down made the discovery, hence it bears his name.
The following are the main signs of down syndrome:
1) A Broad Head
2) Constantly open mouth with tongue sticking out
3) Rounded face (sometimes referred to as Moonface)
4) A downward-facing lip
5) The fold of the Mongolian eyelid
6) Epicanthus with a vertical fold on either side of the snout
7) Narrow Fingers
8) Wide palm with a distinctive palm crease
9) Narrow neck
10) Mental retardation-related low intellect
11) Immature and underdeveloped gonads (testes and ovary)

Remember that the nondisjunction of the 21st pair of chromosomes during the anaphase of the cell cycle results in Down's syndrome. Sex chromosomal diseases include Turner's syndrome and Klinefelter's syndrome.

Hence, option B is the correct option.