**Core Concept**
Male infertility is a complex condition that can be caused by various factors, including low sperm count, abnormal sperm morphology, and impaired sperm motility. The World Health Organization (WHO) has established reference values for semen analysis, which are used to diagnose male infertility.

**Why the Correct Answer is Right**
According to the WHO criteria, a sperm count of less than 15 million sperm per milliliter (mL) of semen is considered below the normal range, and is associated with increased risk of infertility. This is because a normal sperm count is essential for fertilization to occur. The testes produce sperm, which are then transported through the epididymis, vas deferens, and prostate gland before ejaculation. A low sperm count can be caused by various factors, including hormonal imbalances, testicular damage, or genetic disorders.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is missing, so we cannot evaluate it.
* **Option B:** This option is also missing, so we cannot evaluate it.
* **Option C:** This option is not specified, so we cannot provide an explanation for why it is incorrect.
* **Option D:** This option is also not specified, so we cannot provide an explanation for why it is incorrect.

**Clinical Pearl / High-Yield Fact**
It's essential to note that male infertility can be caused by various factors, including hormonal imbalances, testicular damage, or genetic disorders. A thorough evaluation, including semen analysis, hormone testing, and imaging studies, is necessary to diagnose the underlying cause of male infertility.

**Correct Answer:** 15 million
Correct Answer: 15 million sperm per milliliter of semen.