Nondisjunction can happen in either part of meiosis, I or II, but it depends on chromatids not separating, with some for one homolog going to one pole and none to the other. This is, then, about anaphase, either I or II.
Work Step by Step
To understand this, see that nondisjunction is about more chromosomes going to one daughter cell than the other. Since pairs of chromosomes are attached, it must be that poor separation leads to the unequal distribution of chromosomes. This separation occurs in anaphase, which is when the problem must occur. The question indicates that this could be in either part of meiosis, so it could be anaphase I or anaphase II.