Fertilization membrane is formed to

Options

A. Facilitate entry of sperm into egg
B. Provide stability to egg
C. Prevent monospermy
D. Prevent polyspermy.

Answer

### Step-by-Step Solution:

1. **Understanding Fertilization Membrane**: The fertilization membrane is a protective layer that forms around the egg after fertilization. It plays a crucial role in the reproductive process.

2. **Function of Fertilization Membrane**: One of the primary functions of the fertilization membrane is to prevent polyspermy, which is the fertilization of an egg by multiple sperm cells. This is critical because polyspermy can lead to abnormal development of the embryo.

3. **Formation Process**: After a sperm successfully penetrates the egg, the fertilization membrane forms as a result of changes in the egg's outer layers. This process involves the thickening and separation of the vitelline membrane from the surface of the egg.

4. **Conclusion**: Given the options provided, the correct answer is that the fertilization membrane is formed to prevent polyspermy.

### Final Answer:
The fertilization membrane is formed to **prevent polyspermy** (Option D).

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