Final answer:
An organism with three copies of a particular chromosome instead of the normal two is described as trisomic, with Trisomy 21 or Down syndrome being the most well-known example of this condition in humans.
Step-by-step explanation:
An organism that has three copies of a particular chromosome instead of the normal two copies is said to be trisomic. This condition results from a chromosomal abnormality where an individual has an extra chromosome (three instead of the usual pair), leading to an imbalance in gene dosage. An example of trisomy in humans is Trisomy 21, commonly known as Down syndrome, where individuals have an extra copy of chromosome 21.
Individuals with trisomy suffer from various developmental challenges due to the excess of gene products encoded by the extra chromosome. The correct option for the question is c) Trisomic.