**Core Concept**
Oogenesis is the process of formation of a female gamete or ovum from the germ cells in the ovaries, involving meiosis. The first meiotic division is a critical step in this process. **Meiosis I** is characterized by the pairing of homologous chromosomes and their subsequent separation.

**Why the Correct Answer is Right**
The first meiotic division in oogenesis results in the formation of a **secondary oocyte** and the first polar body. This division is unique because it is arrested at the diplotene stage of prophase I until ovulation occurs. Upon ovulation, **Meiosis I** completes, leading to the formation of a secondary oocyte and the first polar body, each containing a haploid set of chromosomes.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the first meiotic division does not complete to form a mature ovum.
**Option B:** Incorrect as it is not directly related to the completion of the first meiotic division.
**Option D:** Incorrect because it refers to a later stage in the process.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the first meiotic division is completed at ovulation, resulting in a **secondary oocyte**. This understanding is crucial for appreciating reproductive physiology and pathology.

**Correct Answer:** C. at ovulation