Final answer:
A human zygote with 45 chromosomes would most likely result from nondisjunction in meiosis I, an error that leads to the production of gametes with an abnormal number of chromosomes, causing aneuploidy such as monosomy when fertilized.
Step-by-step explanation:
The process most likely to produce a human zygote with 45 chromosomes is A) Nondisjunction in meiosis I. Nondisjunction is an error during meiosis where homologous chromosomes or sister chromatids fail to separate properly. If nondisjunction occurs during meiosis I, all resulting gametes from this division will either have an extra chromosome or be missing one (n+1 or n-1), leading to a condition known as aneuploidy. When such a gamete fuses with a normal gamete during fertilization, the resulting zygote will have an abnormal chromosome number, such as 45 or 47, instead of the typical 46. This could result in monosomy if a chromosome is missing or trisomy if there is an extra chromosome. In contrast, nondisjunction during meiosis II does not affect all gametes, resulting in some normal gametes and some with an abnormal chromosome number.