**Core Concept**
In a healthy adult, the blood composition is a delicate balance of different cell types, each playing a crucial role in the body's defense mechanisms and overall health. The normal distribution of white blood cells, also known as leukocytes, is a key aspect of this balance.

**Why the Correct Answer is Right**
Lymphocytes are a type of white blood cell that plays a central role in the immune system. They are responsible for recognizing and responding to specific pathogens, such as bacteria and viruses. In a normal adult, lymphocytes typically make up around 30% of the total white blood cell count, with the majority being T lymphocytes (around 70%) and B lymphocytes (around 20-30%). This distribution allows for an effective immune response to various infections and diseases.

**Why Each Wrong Option is Incorrect**
**Option A:** 40% neutrophils is incorrect because, although neutrophils are the most abundant type of white blood cell, they typically make up around 50-70% of the total white blood cell count, not 40%.

**Option C:** 20% eosinophils is incorrect because eosinophils are a type of white blood cell that plays a key role in fighting parasites and in allergic reactions. However, they typically make up only around 1-4% of the total white blood cell count in a normal adult.

**Option D:** 10% basophils is incorrect because basophils are a type of white blood cell that plays a key role in inflammation and allergic reactions. However, they typically make up only around 0.5-1% of the total white blood cell count in a normal adult.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the normal distribution of white blood cells can vary depending on various factors, such as age, health status, and exposure to infections. For example, in children and infants, lymphocytes may make up a higher percentage of the total white blood cell count.

**✓ Correct Answer: B. 30% lymphocytes**