Monosomy or trisomy occurs because of a. crossing-over. b. inversion. c. translocation. d. nondisjunction.

We have four options related to chromosomal activities or abnormalities: a. Crossing-over: It is the exchange of genetic material between homologous chromosomes during meiosis, resulting in new combinations of alleles. b. Inversion: It is a chromosomal abnormality in which a segment of a chromosome is reversed end-to-end and reinserted into the same chromosome. c. Translocation: It is a chromosomal abnormality where a segment of genetic material from one chromosome is transferred to another chromosome, resulting in an altered arrangement of genes on the chromosomes. d. Nondisjunction: This occurs when homologous chromosomes or sister chromatids fail to separate properly during meiosis, resulting in an unequal distribution of chromosomes in the resulting gametes.

Now, let's consider each option and decide which one is responsible for monosomy or trisomy: a. Crossing-over: It is not responsible for monosomy or trisomy, as it results in new combinations of alleles without having an effect on the chromosome count. b. Inversion: Similar to crossing-over, inversion results in changes in gene arrangement without affecting the overall count of chromosomes; thus, it is not responsible for monosomy or trisomy. c. Translocation: While this chromosomal abnormality can change the arrangement of genes, it does not increase or decrease the overall count of chromosomes. So, it's not responsible for monosomy or trisomy either. d. Nondisjunction: As nondisjunction causes the uneven distribution of chromosomes in gametes, it leads to improper chromosome counts in the resulting offspring. If a gamete with an extra chromosome fertilizes a normal gamete, the offspring would have a trisomy (an extra chromosome). Alternatively, if a gamete with one fewer chromosome fertilizes a normal gamete, the offspring would have a monosomy (one less chromosome).

Based on the analysis of the four given options, we can conclude that monosomy or trisomy occurs because of: d. Nondisjunction