**Core Concept**
The question is testing the student's knowledge of different types of lymphocytes, which are a crucial part of the immune system. Lymphocytes are a type of white blood cell that plays a key role in the immune response, and they can be broadly classified into three main categories: T lymphocytes (T cells), B lymphocytes (B cells), and natural killer cells (NK cells).

**Why the Correct Answer is Right**
NK cells are a type of lymphocyte that is distinct from T and B cells. They are involved in the innate immune response and are capable of killing tumor cells and virus-infected cells without prior antigen exposure. NK cells recognize and destroy cells that are missing or have reduced expression of major histocompatibility complex (MHC) class I molecules, which are usually expressed on the surface of healthy cells. This unique function of NK cells makes them an essential part of the immune system.

**Why Each Wrong Option is Incorrect**
**Option A:** Macrophages are a type of white blood cell, but they are not a class of lymphocytes. They are involved in the phagocytosis of foreign particles and cellular debris, and they also play a role in the presentation of antigens to T cells.

**Option B:** Astrocytes are a type of glial cell found in the central nervous system, and they have nothing to do with the immune system or lymphocytes.

**Option D:** Langerhans cells are a type of dendritic cell found in the skin and other mucosal surfaces. They are involved in the presentation of antigens to T cells, but they are not a class of lymphocytes.

**Clinical Pearl / High-Yield Fact**
NK cells are often referred to as "natural killer" cells because they can recognize and kill tumor cells and virus-infected cells without prior antigen exposure. This unique function makes them an important part of the immune system, and they are being studied as potential targets for cancer immunotherapy.

**✓ Correct Answer:** C. NK cells