**Core Concept**
The average reproductive life span of an ovum refers to the length of time a mature ovum remains fertile and capable of fertilization after ovulation. This concept is crucial in understanding human reproduction and the timing of conception.

**Why the Correct Answer is Right**
The correct answer is 12-24 hours. This duration is critical because it allows for the fertilization of the ovum by sperm and subsequent implantation of the zygote in the uterine lining. The average lifespan of a mature ovum is around 12-24 hours after ovulation, after which it degenerates and becomes non-fertile. This brief window of fertility emphasizes the importance of timing in human reproduction.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
The 6-day menstrual cycle is divided into two main phases: the follicular phase (days 1-14) and the luteal phase (days 15-28). Understanding the reproductive life span of an ovum is essential for calculating the optimal timing for conception and fertility treatments.

**Correct Answer: D. 12-24 hours**