At fertilization

Explanation

During fertilization, the sperm cell from the male and the egg cell from the female combine to form a zygote, which will eventually develop into a new organism. Fertilization involves the fusion of the male and female gametes, resulting in the formation of a single cell with the full complement of chromosomes. The question asks specifically about what happens during fertilization. Option A states that one chromosome from the male joins another from the female. While this is true, it is not a complete answer to the question. Option B describes gene combinations, which are important but not the main process that occurs during fertilization. Option D suggests the combination of chromosome sets, but this is not an accurate description of fertilization. The correct answer is option C, which states that the male nucleus fuses with the female nucleus. This is the key process that occurs during fertilization, where the haploid nuclei of the sperm and egg cells combine to form a diploid zygote with a complete set of chromosomes. In summary, during fertilization, the male and female gametes fuse together to form a zygote. The correct answer to the question is option C, which describes the fusion of the male and female nuclei.