**Core Concept**
The World Health Organization (WHO) has established a set of criteria to define normal sperm parameters, which are crucial for assessing male fertility. These criteria are based on extensive research and are widely accepted in the medical community.

**Why the Correct Answer is Right**
The correct answer, 20 million/ml, is the WHO's definition of normal sperm count. This value represents the minimum number of sperm per milliliter of semen that is considered normal for fertility. The WHO's criteria also include other parameters, such as sperm morphology and motility, but sperm count is a critical factor in evaluating male fertility. The WHO's definition is based on the understanding that a sperm count of 20 million/ml or higher is associated with a higher likelihood of successful fertilization.

**Why Each Wrong Option is Incorrect**
**Option A:** 10 million/ml is below the WHO's definition of normal sperm count and is associated with reduced fertility. Men with sperm counts below 10 million/ml may experience difficulty conceiving.

**Option C:** 40 million/ml is higher than the WHO's definition of normal sperm count, but it does not provide any additional benefits for fertility. Sperm counts above 40 million/ml are not typically associated with improved fertility outcomes.

**Option D:** 50 million/ml is not a recognized definition of normal sperm count by the WHO or any other reputable medical organization.

**Clinical Pearl / High-Yield Fact**
It's essential to note that sperm count is not the only factor that determines fertility. Other parameters, such as sperm morphology, motility, and DNA integrity, also play critical roles in determining a man's fertility potential.

**✓ Correct Answer: B. 20 million/ml**