Which of the following best describes an abnormal number of chromosomes?

Aneuploidy refers to an abnormal number of chromosomes within a cell, which can occur when there is an addition or loss of chromosomes from the typical diploid set. For instance, a cell with 47 chromosomes instead of the usual 46 is an example of aneuploidy. This condition can result from errors during cell division, specifically in the processes of mitosis or meiosis, leading to genetic disorders such as Down syndrome, which is caused by the presence of an extra copy of chromosome 21.

In contrast, euploidy denotes a normal set of chromosomes. Diploidy refers specifically to the condition of having two complete sets of chromosomes, which is typical for most somatic cells in diploid organisms. Haploidy describes a single set of chromosomes, which is characteristic of gametes (sperm and eggs) in sexual reproduction. These definitions illustrate that while euploidy, diploidy, and haploidy denote normal chromosome number scenarios, aneuploidy signifies a deviation from this norm, identifying it as the correct choice for the question.