**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 characterized by their ability to recognize and destroy cells that are not recognized as self, without prior antigen exposure.

**Why the Correct Answer is Right**
NK cells are derived from the hematopoietic stem cells in the bone marrow and are a key component of the innate immune response. They recognize and target cells that lack major histocompatibility complex (MHC) class I molecules, which are typically expressed on the surface of normal cells. This recognition is mediated by activating and inhibitory receptors on the surface of NK cells, which interact with ligands on the target cell. When the activating signals outweigh the inhibitory signals, NK cells are triggered to release cytotoxic granules that kill the target cell.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because NK cells are not primarily involved in the adaptive immune response, which requires antigen presentation and T-cell activation.
**Option B:** Incorrect because NK cells are not a type of B-cell, which is a type of lymphocyte involved in the production of antibodies.
**Option C:** Incorrect because NK cells are not a type of T-cell, which is a type of lymphocyte involved in cell-mediated immunity.

**Clinical Pearl / High-Yield Fact**
NK cells play a crucial role in the defense against viral infections, particularly in the early stages of infection. They recognize and destroy infected cells before the virus has a chance to replicate and cause widespread damage.

**Correct Answer: C. Natural killer cells.**