## Core Concept
Natural Killer (NK) cells are a type of lymphocyte that play a crucial role in the innate immune system. They are involved in the defense against viral infections and in the surveillance against tumor cells. NK cells can recognize and kill certain types of cells without prior sensitization.

## Why the Correct Answer is Right
The correct statement about NK cells not listed here requires identifying incorrect statements among the options. Generally, NK cells are known for their ability to recognize stressed cells in the absence of antibodies and MHC, utilizing activating and inhibitory receptors.

## Why Each Wrong Option is Incorrect
- **Option A:** Typically, NK cells do not require prior antigen exposure to kill their targets, which makes them part of the innate immune response. If option A suggests they need prior exposure, it's incorrect.
- **Option B:** If option B states that NK cells can kill tumor cells and virus-infected cells, this would be a true statement regarding their function.
- **Option C:** If option C mentions that NK cells express CD56 and CD16, this is true as these are markers commonly used to identify NK cells.
- **Option D:** Without the specific details of each option, we consider a generic incorrect statement: If an option claims NK cells are primarily involved in adaptive immunity or require MHC class II for activation, it would be incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that NK cells are crucial for controlling viral infections and tumor surveillance. They can act quickly to eliminate infected cells or tumor cells without the need for prior antigen exposure, making them vital for early defense.

## Correct Answer: D.