Which condition is characterized by an abnormal number of chromosomes in a cell?

The condition characterized by an abnormal number of chromosomes in a cell is aneuploidy. This term refers to the presence of an abnormal number of chromosomes, which can manifest as either an excess or a deficit compared to the standard diploid number. Aneuploidy can lead to various developmental and health issues, including genetic disorders such as Down syndrome, which is caused by an extra copy of chromosome 21.

The other options refer to different biological processes or conditions. Hyperplasia involves an increase in the number of cells in a tissue or organ, often leading to its enlargement. Hypoplasia refers to the underdevelopment or incomplete development of a tissue or organ, resulting in fewer cells than normal. Angiogenesis is the process through which new blood vessels form from existing ones. Each of these terms relates to cellular growth or development, rather than chromosome count, distinguishing them from aneuploidy.