## Core Concept
Natural Killer (NK) cells are a type of lymphocyte that play a critical role in the innate immune system. They are known for their ability to recognize and destroy tumor cells and virus-infected cells without prior sensitization. NK cells act through various mechanisms, including the release of cytotoxic granules and the production of cytokines.

## Why the Correct Answer is Right
The correct answer, although not directly provided, relates to understanding the characteristics and functions of NK cells. Typically, NK cells are described as CD56+ and CD3-, distinguishing them from T cells which are CD3+. They can kill target cells through mechanisms that do not require antigen presentation, a key feature distinguishing them from adaptive immune responses.

## Why Each Wrong Option is Incorrect
- **Option A:** Without the specific details of option A, we can infer that any statement claiming NK cells require prior antigen exposure to function or stating they are part of the adaptive immune response would be incorrect. NK cells are part of the innate immune system and do not require prior sensitization to kill target cells.
- **Option B:** Similarly, without specifics, if option B suggests NK cells are ineffective against tumor cells or virus-infected cells, this would be incorrect. NK cells are known for their role in surveillance against tumor cells and viral infections.
- **Option C:** If option C mentions that NK cells express CD3, this would be incorrect. NK cells are typically characterized by their expression of CD56 and the lack of CD3, a marker for T cells.
- **Option D:** Assuming option D provides an accurate description of NK cell function or characteristic, it would be the correct statement about NK cells.

## Clinical Pearl / High-Yield Fact
A key point to remember is that NK cells are crucial in the body's defense against viral infections and in the surveillance against tumor cells. Their function can be enhanced by certain cytokines, such as IL-2. A classic clinical correlation is the observation of decreased NK cell activity in individuals with certain immunodeficiencies.

## Correct Answer: D.