The correct option is "when the sperm gains entry into the cytoplasm of the ovum".

To understand why this is correct, let's break down the stages of oogenesis and fertilization in humans step-by-step:
1. Arrest at Prophase I: Oogenesis begins during embryonic development. Primary oocytes start meiosis I but remain suspended in the diplotene stage of prophase I until puberty.
2. Arrest at Metaphase II: Prior to ovulation, a primary oocyte completes its first meiotic division (meiosis I) to form a secondary oocyte and a first polar body. The secondary oocyte then initiates the second meiotic division (meiosis II) but is arrested once again, specifically at the metaphase II stage.
3. Ovulation: It is this arrested secondary oocyte that is released from the ovary during ovulation and travels down the Fallopian tube.
4. Trigger for Completion: The secondary oocyte will not complete meiosis II unless it is fertilized. When a sperm successfully penetrates the outer layers (corona radiata and zona pellucida) and its membrane fuses with that of the secondary oocyte, gaining entry into the oocyte's cytoplasm, it triggers a calcium-ion wave.
5. Completion of Meiosis II: This signal induces the activation of anaphase-promoting complex, which allows the secondary oocyte to resume and complete its second meiotic division. This division is unequal, producing a large ootid (which quickly matures into the functional ovum) and a tiny second polar body.

Therefore, meiosis II is completed specifically when the sperm gains entry into the cytoplasm of the ovum, leading to the formation of a diploid zygote once the haploid nuclei fuse.