In the process of fertilization, this is true

The correct option is "entry of sperm activates the egg for completing meiosis".

During human oogenesis, the egg (oocyte) is arrested at the metaphase II stage of meiosis II. It remains in this suspended state of division until fertilization occurs.
When a sperm binds to and penetrates the egg's plasma membrane, it triggers a depolarization and a wave of calcium ion release within the egg. This increase in intracellular calcium acts as a chemical signal that activates the egg, releasing it from its meiotic arrest. Consequently, the egg completes its second meiotic division (meiosis II), producing a mature ovum and extruding the second polar body.

Let's evaluate why the other options are incorrect to understand the process of fertilization fully:
• "only one sperm reaches the egg and enters it": While typically only one sperm successfully fertilizes the egg (due to blocks against polyspermy), millions of sperm travel toward the egg, and many may reach the egg's outer layers (corona radiata and zona pellucida) to assist in breaking them down, even though only one enters.
• "only the acrosome of the sperm enters the egg": The acrosome is a cap-like structure containing enzymes that help penetrate the egg's outer coats. During fertilization, the sperm head (containing the nucleus) and the middle piece/centriole enter the oocyte cytoplasm, not just the acrosome content.
• "two haploid nuclei fuse and immediately divide to produce two nuclei which are again haploid": When the haploid male and female pronuclei fuse (syngamy), they form a single diploid nucleus (zygote) with a restored chromosome count (2n), rather than immediately dividing back into haploid nuclei.