Answer:
The correct answer would be C) extra chromosome; nondisjunction.
Karyotype is the diagrammatic representation of the number and appearance of the chromosomes of an organism.
The normal karyotype of a human being consists of 23 pairs of chromosomes.
In the given karyotype shows trisomy of chromosome 18 that is, it contains an extra copy of the chromosome.
Extra copy of a chromosome is inherited as a result of nondisjunction (failure of separation of sister chromatids or homologous chromosomes) during gamete formation.