**Core Concept**
Natural killer (NK) cells play a crucial role in the innate immune system, recognizing and eliminating aberrant or infected cells. Their primary function is to target and destroy cells that have lost or downregulated their major histocompatibility complex (MHC) class I expression. MHC class I molecules are essential for presenting endogenously synthesized peptides to cytotoxic T cells, and their loss is often associated with viral infections, tumor transformation, or other pathological processes.

**Why the Correct Answer is Right**
NK cells utilize a balance of inhibitory and activating signals to determine whether a cell should be targeted for destruction. Inhibitory signals are triggered by the binding of NK cell receptors to MHC class I molecules on the surface of healthy cells. If a cell is unable to express MHC class I, the inhibitory signals are diminished, and the activating signals predominate, leading to NK cell activation and target cell lysis. This mechanism is essential for eliminating virus-infected cells or tumor cells that have downregulated MHC class I expression to evade immune detection.

**Why Each Wrong Option is Incorrect**
**Option A:** Cells which express MHC 1 are generally not targeted by NK cells, as they receive inhibitory signals that prevent NK cell activation. MHC class I expression is a sign of healthy cellular function, and its presence typically suppresses NK cell-mediated cytotoxicity.
**Option C:** MHC cells which express MHC 2 are not typically targeted by NK cells. MHC class II molecules are primarily involved in presenting exogenously derived antigens to T helper cells, and their expression is not directly related to NK cell activation.
**Option D:** The statement "Cells which are not able to express MHC" is too vague, as it does not specify the class of MHC molecules. However, even if it referred to MHC class I, it would not be entirely accurate, as cells that are unable to express MHC class I are indeed targeted by NK cells.

**Clinical Pearl / High-Yield Fact**
A key aspect of NK cell function is their ability to adapt to changing cellular environments. In the context of viral infections or tumor transformation, NK cells can rapidly adjust their response to eliminate cells that have downregulated MHC class I expression. This dynamic interplay between NK cells and their targets is a critical aspect of the innate immune response.

**✓ Correct Answer: B. Cells which are not able to express MHC 1**