The karyotype of one species of primate has 48 chromosomes. In a particular female, cell division goes awry and she produces one of her eggs with an extra chromosome (25). The most probable source of this error would be a mistake in which of the following?
A) mitosis in her ovary
B) metaphase I of one meiotic event
C) telophase II of one meiotic event
D) telophase I of one meiotic event
E) either anaphase I or II

During anaphase I the homologous chromosomes separate and the two daughter cells will have reduced by half the chromosome number, but each chromosome still has two chromatids.
During anaphase II the chromatids separate, producing daughter cells with one chromatid per chromosome and half the chromosome number as the original diploid cell.

If a mistake happens in anaphase I and two homologous chromosomes migrate together to one pole, one of the daughter cells will have an extra chromosome and the other will be missing one. After meiosis II, two of the four daughter cells will have an extra chromosome and two of them will be missing one.

If a mistake happens in anaphase II the chromatids of one of the chromosomes will fail to separate, and the end result of meiosis will be 2 normal gametes, one with an extra chromosome and one missing a chromosome.