A human ovum completes its second meiosis :

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

Step-by-Step Educational Explanation:

1. The State of the Released Ovum:
During ovulation, the female gamete is released from the ovary as a secondary oocyte, not a fully mature ovum. At this stage, the cell has initiated its second meiotic division (meiosis II) but is arrested mid-process at the metaphase II stage. It will remain in this arrested state unless fertilization occurs.

2. The Fertilization Process:
When sperm surround the secondary oocyte, they must first penetrate the outer cellular layer (corona radiata) and the thick glycoprotein coat (zona pellucida). The acrosome reaction releases enzymes to help the sperm digest a path through these layers. However, these initial interactions (such as touching the zona pellucida or releasing acrosomal enzymes) do not signal the egg to complete cell division.

3. Triggering the Completion of Meiosis II:
Once a single sperm successfully passes through the zona pellucida, its plasma membrane fuses with the plasma membrane of the secondary oocyte. This fusion allows the sperm to gain entry into the cytoplasm of the ovum. The entry of the sperm initiates a rapid release of calcium ions within the oocyte's cytoplasm. This calcium wave acts as the physiological signal that releases the cell from its metaphase II arrest, leading to the rapid completion of meiosis II.

4. Final Outcome:
The completion of meiosis II results in an unequal division of the cytoplasm, yielding a large haploid mature ovum (ootid) containing the maternal pronucleus and a tiny, non-functional cell called the second polar body. The maternal and paternal pronuclei then fuse to form the diploid zygote.