**Core Concept**
Oogenesis is the process of female gamete formation in the ovaries. It involves meiotic division of a primary oocyte into a mature ovum (oocyte) and a polar body. Meiosis II is the final stage of oogenesis, resulting in the separation of the chromosomes into the mature ovum and a second polar body. The completion of meiosis II marks the end of oogenesis, and the resulting mature ovum is ready for fertilization.

**Why the Correct Answer is Right**
Immediately following the completion of meiosis II, the mature ovum, now containing the diploid number of chromosomes, begins to form a female pronucleus. This is a critical step in the process of fertilization, where the genetic material of the sperm and the ovum will combine to form a zygote. The formation of the female pronucleus involves the breakdown of the nuclear membrane and the condensation of the chromatin into a visible nucleus. This process allows for the subsequent fusion of the sperm and ovum nuclei during fertilization. The formation of the female pronucleus is a key event in the preparation of the ovum for fertilization.

**Why Each Wrong Option is Incorrect**
**Option A:** Degeneration of the zona pellucida occurs after fertilization, when the sperm penetrates the ovum. It is not a direct consequence of the completion of meiosis II.
**Option B:** Sperm penetration of the corona radiata is a separate event that occurs before fertilization, when the sperm reaches the ovary and penetrates the outer layers of the ovum.
**Option D:** The appearance of the blastocyst occurs several days after fertilization, when the zygote undergoes several cleavage divisions and forms a blastocyst, which then implants in the uterus.

**Clinical Pearl / High-Yield Fact**
The formation of the female pronucleus is a critical step in the process of fertilization, and its proper completion is essential for the formation of a viable zygote.

**✓ Correct Answer: C. Formation of a female pronucleus**