In Down Syndrome, there is the presence of an extra copy of chromosome 21, which means individuals have three copies (trisomy 21) instead of the usual two. This chromosomal anomaly leads to the characteristic features and developmental delays associated with the condition.

To clarify the options:
- Chromosome 21 is the one affected in Down Syndrome.
- Chromosome 18 trisomy causes Edwards syndrome.
- Chromosome 13 trisomy leads to Patau syndrome.
- Chromosome X abnormalities relate to sex chromosome disorders such as Turner syndrome or Klinefelter syndrome, but are not related to Down Syndrome.

Thus, the correct answer is Chromosome 21 because the presence of an extra chromosome 21 causes the genetic and phenotypic manifestations of Down Syndrome.

Key Points:
- Down Syndrome results from trisomy 21 (three copies of chromosome 21).
- It is the most common chromosomal cause of intellectual disability.
- Different trisomies cause distinct syndromes (e.g., 18 for Edwards, 13 for Patau).

Reference: Robbins Basic Pathology, 10th Edition, Chapter 5: Genetic Disorders, Page 158