Okay, let's tackle this question about fertilization. The question is asking when fertilization is considered complete. I need to recall the steps of the fertilization process.

First, fertilization involves the union of a sperm and an egg. The initial step is the sperm penetrating the zona pellucida of the egg. Then, the acrosomal reaction occurs, where enzymes from the sperm help it reach the egg's membrane. Once the sperm enters the egg, the egg's membrane changes to prevent other sperm from entering, which is the cortical reaction.

The next key step is the formation of the male and female pronuclei. The sperm's nucleus becomes the male pronucleus, and the egg's nucleus becomes the female pronucleus. Fertilization is considered complete when these two pronuclei fuse. This fusion leads to the formation of the zygote.

So the correct answer should be when the pronuclei fuse. Let me check the options again. The options are missing, but the correct answer is likely about pronuclear fusion. Common distractors might include the acrosomal reaction, sperm entry, or the cortical reaction. Each of these happens before the actual fusion of pronuclei.

The clinical pearl here is that pronuclear fusion marks the completion of fertilization. Students often confuse the steps, so it's important to remember that the final step is the formation of the zygote through pronuclear fusion.

**Core Concept**
Fertilization is the process where male and female gametes unite to form a zygote. It is complete when the genetic material from both gametes (male and female pronuclei) fuses, initiating zygotic development.

**Why the Correct Answer is Right**
Fertilization is complete when the **male and female pronuclei fuse**, forming a diploid zygote. This occurs after sperm penetration, acrosomal reaction, and cortical reaction. Pronuclear fusion ensures the combination of paternal and maternal genomes, which is essential for embryonic development. This step follows the formation of the fertilization membrane and is distinct from earlier events like sperm binding or acrosome reaction.

**Why Each Wrong Option is Incorrect**
**Option A:** Sperm binding to the zona pellucida is the initial step, not completion.
**Option B:** The acrosomal reaction enables sperm to penetrate the zona pellucida but does not complete fertilization.
**Option C:** The cortical reaction prevents polyspermy but precedes pronuclear fusion.

**Clinical Pearl / High-Yield Fact**
Remember: Fertilization is *not* complete until pronuclear fusion occurs. Confusing this with earlier steps (e.g., sperm entry) is a common exam trap. Use the mnemonic **"Pronuclei = Pregnancy starts"** to recall this key point.

**Correct Answer: C. Fusion of male and female pronuclei**

✓ Correct Answer: B. 2nd polar body is formed