Final answer:
A human zygote with 45 chromosomes can result from fusion of an egg with two sperm cells or a genetic mutation during cell division.
Step-by-step explanation:
A human zygote with 45 chromosomes can result from b) Fusion of an egg with two sperm cells or c) A genetic mutation during cell division.
In option b), if an egg is fertilized by two sperm cells instead of one, it will result in a zygote with three sets of chromosomes, totaling 45 chromosomes. This condition is known as triploidy.
In option c), a genetic mutation during cell division can result in a zygote with a missing chromosome, resulting in a total of 45 chromosomes instead of the usual 46. This condition is known as monosomy.