**Core Concept**
Natural Killer (NK) cells are a type of lymphocyte that plays a crucial role in the innate immune system, particularly in the defense against viral infections and tumor cells. They are capable of recognizing and eliminating infected cells or tumor cells without prior antigen exposure.

**Why the Correct Answer is Right**
NK cells recognize and kill target cells through a mechanism known as the "missing self" hypothesis, where they bind to MHC class I molecules that are either absent or down-regulated on the surface of infected or tumor cells. This interaction activates the NK cell, leading to the release of cytotoxic granules that contain proteins such as perforin and granzymes, which ultimately cause cell death. The Fc receptor (CD16) on NK cells also enables them to engage in antibody-dependent cellular cytotoxicity (ADCC), where they can recognize and destroy cells bound by antibodies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because NK cells do not primarily function through the complement system, which is a separate component of the innate immune system.
**Option B:** This option is incorrect because NK cells do express CD4 and CD8 co-receptors, but they are not the primary receptors involved in their activation and function.
**Option C:** This option is incorrect because NK cells are not the only type of lymphocyte capable of recognizing and killing tumor cells; T cells and macrophages also play a role in this process.

**Clinical Pearl / High-Yield Fact**
NK cells can be activated by cytokines such as IL-2 and IL-12, which can enhance their cytotoxic activity and play a crucial role in the immune response against viral infections and tumors.

**Correct Answer:** Not provided. Please provide the options and the correct answer so I can complete the explanation.