**Core Concept**
The zona pellucida is a glycoprotein layer surrounding the oocyte and early embryo, providing structural support and protecting the embryo. It is essential for sperm binding and fertilization but must be degraded to allow embryonic cell division and implantation.

**Why the Correct Answer is Right**
After fertilization, the zona pellucida undergoes a process called "zona hardening" followed by breakdown. The enzyme **proteolytic activity** of the embryo, primarily via **proteases like hyaluronidase**, begins to degrade the zona pellucida. This degradation is completed by the **5th day post-fertilization** when the blastocyst forms and the zona begins to thin. This timing ensures that the embryo is no longer protected by the zona, allowing it to implant in the uterine lining.

**Why Each Wrong Option is Incorrect**
Option A: Before fertilization – Incorrect because the zona pellucida is present at fertilization and is essential for sperm-egg binding.
Option B: During fertilization – Incorrect because while fertilization triggers zona hardening, the zona does not disappear at this stage; it remains intact until later.
Option D: 7th day after fertilization – Incorrect because by the 7th day, the embryo has already implanted and the zona has already been degraded by the 5th day.

**Clinical Pearl / High-Yield Fact**
In assisted reproductive technologies (ART), such as IVF, the zona pellucida must be intact until the blastocyst stage to allow embryo transfer. If the zona is absent or too thin, it can lead to failed implantation or loss of embryo viability.

✓ Correct Answer: C. 5th day after feilization