**Core Concept**
The question pertains to the menstrual cycle, specifically the ovarian cycle, where oocytes undergo maturation. The correct answer will highlight the characteristics of a secondary oocyte, which is a critical stage in the reproductive process.

**Why the Correct Answer is Right**
A secondary oocyte is a mature oocyte that has completed meiosis I but not meiosis II. The first meiotic division in females occurs during fetal development, resulting in a primary oocyte. At puberty, the primary oocyte resumes meiosis I, but it arrests at metaphase II. The secondary oocyte is released from the ovary during ovulation and can be fertilized by a sperm. If fertilization occurs, the secondary oocyte completes meiosis II, resulting in a mature ovum.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because a primary oocyte is an immature oocyte that has not yet undergone meiosis I.
**Option B:** Incorrect because a mature ovum is the result of complete meiosis II, not the definition of a secondary oocyte.
**Option C:** Incorrect because a graafian follicle is the mature follicle that releases a secondary oocyte during ovulation, but it is not the secondary oocyte itself.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a secondary oocyte can only be fertilized if it is released from the ovary during ovulation and then undergoes the second meiotic division after fertilization.

**Correct Answer:** D. Secondary oocyte.