Final answer:
Meiosis II is completed only after and because of fertilization, wherein the secondary oocyte progresses to form a haploid ovum and a diploid zygote when combined with sperm.
Step-by-step explanation:
The process of oogenesis involves the transformation of a diploid secondary oocyte into a haploid ovum. This critical event known as meiosis II is completed after and only if the secondary oocyte is fertilized by a sperm. Once fertilized, the secondary oocyte quickly progresses through meiosis II, producing a haploid ovum and a second polar body, which later degenerates. This step is pivotal as it allows the oocyte to fuse with the (haploid) sperm, creating a diploid zygote, which is the first cell of a new organism. If fertilization does not take place, the secondary oocyte does not complete meiosis II and disintegrates.
Clarifying the question, the response is: B. After fertilization.
This process is essential because the resulting zygote receives all of its cytoplasm from the ovum, necessitating that the egg contains as much cytoplasm as possible to support early embryonic development. Furthermore, the completion of meiosis II ensures the proper haploid state of the ovum, preparing it for the fusion of genetic materials from both the mother and the father to form a unique genetic profile in the resultant zygote.