The correct answer is:
d - Either the sperm cell or the egg cell has two copies of chromosome 21.
This occurs due to a mistake in meiosis, where the process of gamete formation results in one of the gametes (sperm or egg) having an extra copy of chromosome 21. When this gamete fuses with a normal gamete, the resulting fertilized egg has three copies of chromosome 21, which leads to Down's Syndrome, also known as Trisomy 21.