**Core Concept**
Null cells, also known as natural killer cells, are a type of lymphocyte that plays a crucial role in the innate immune response. They do not express specific antigen receptors and do not undergo clonal expansion like T and B cells. Null cells are an essential component of the lymphocyte population, contributing to the body's defense against viral infections and tumor cells.

**Why the Correct Answer is Right**
Null cells are indeed a significant portion of the total lymphocyte population. They are characterized by their ability to recognize and destroy infected cells or tumor cells without prior antigen exposure. Null cells express Fc receptors for IgG and can be activated through antibody-dependent cellular cytotoxicity. The exact percentage of null cells can vary depending on the individual and the specific immune response, but they generally constitute a substantial fraction of the total lymphocyte count.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too low; null cells typically make up a larger percentage of the total lymphocyte population.
**Option B:** This option is incorrect because it is too high; while null cells are a significant portion of the total lymphocyte population, they do not make up the majority.
**Option D:** This option is incorrect because it is not a valid percentage; the correct answer should be a specific numerical value between 0 and 100.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that null cells are an important component of the innate immune response and play a crucial role in the defense against viral infections and tumor cells. Understanding their function and percentage in the total lymphocyte population can help clinicians diagnose and manage various immune-related disorders.

**Correct Answer:** C. 20-30%.