The failure of homologous chromosomes to separate properly during meiosis
(a) nondisjunction.
(b) disjunction.
(c) zygote failure.
(d) telomere failure

nondisjunction.

Explanation:

In meiotic division, the homologous chromosomes divide during anaphase I. This results formation of tetrads each containing haploid chromosomes from 2n chromosomes. Sometimes the homologous chromosomes fail to separate which results in the nondisjunction of the chromosomes. In the first phase of meiosis if the homologous chromosomes fail to separate, then one daughter cell has 2 homologous chromosome and others have no chromosome. When the anaphase 1 completes the 4 daughter cells formed, out of which 2 daughter cells contain 2 sister chromatids and the other 2 are without any chromosome. When the fusion of gametes occurs 2 types of cells form 2 cells having 3 chromosomes and the other 2 have one diploid chromosome. The cells having 3 chromosomes are known as trisomic and the cell having 1 chromosome called monosomic. This condition is called trisomy and monosomy.

If the nondisjunction happens in anaphase 2 in the meiosis, in the first phase of anaphase 2 daughter cells have 2 homologous chromosomes. In the second phase of the anaphase, the homologous chromosomes separate in one cell and others don't separate. When the tetrads form 2 daughter cells have a normal haploid chromosome. The remaining 2 daughter cells one has 2 haploid chromosomes and others have no chromosome. When these cells fuse with other haploid cells 2 cells will be normal having diploid chromosomes. Out of the 2 remaining cells, one cell has 3 chromosomes other have one. It is a 50 - 50 condition where 2 are normal diploid cell after fusion and other 2 are form trisomy and monosomy condition.

A) nondisjunction

Explanation:

When chromosomes  fail to separate from each other during meiosis meiotic nondisjunction occurs. If the failure to  separate occurs during the first meiotic division or in the parent generation, it is called primary  nondisjunction. The haploid cells will have an abnormal number of  chromosomes then. If nondisjunction happens at the second meiotic  division, it is called secondary nondisjunction. Then only two of the four  haploid cells are affected.