Concept:Fertilization is the union of the male and female gamete nuclei to form a single diploid cell called the zygote.
Explanation:When a sperm meets an egg, only the head of the sperm, which contains the nucleus, enters the egg cell.
The tail of the sperm remains outside and is left behind; it does not enter the egg.
Inside the egg, the sperm nucleus fuses with the egg nucleus.
This fusion of the two haploid nuclei restores the diploid chromosome number and produces the zygote.
Therefore, the essential process that takes place during fertilization is the fusion of the sperm nucleus with the egg nucleus, not the fusion of the whole sperm or the tail.
Answer:C. The nucleus of the sperm fuses with the egg cell.