If this is the picture you are talking about, the right answer is non-disjunction
Non-disjunction is the non-separation of homologous chromosomes at the time of cell division which results in the formation of ova or spermatozoa leading to an abnormality in the number of chromosomes of the egg thus fertilized by the spermatozoa.
The fertilized egg consists of either a single chromosome, what is called monosomy, or, conversely, three chromosomes, this is called trisomy. While naturally, the fertilized egg has a single pair of chromosomes.
