Polyploidy is a condition where an organism has more than two complete sets of chromosomes. This can occur due to errors in cell division, such as nondisjunction, where chromosomes fail to separate properly. The provided correct answer states that polyploidy arises due to a change in the number of chromosomes. This is accurate because polyploidy is characterized by an increase in the number of chromosomes, which can result in an organism having three or more complete sets of chromosomes instead of the usual two sets. Options 1 and 2 are incorrect as polyploidy is not caused by changes in the number of chromatids or the structure of genes. Option 4 is also incorrect as polyploidy is not caused by changes in the structure of chromosomes.