## **Core Concept**
The question targets the understanding of immune cells' roles in combating viral infections and cancer. The immune system employs various cell types to defend against pathogens and abnormal cells. Among these, certain cells are pivotal in directly acting against viruses and cancer cells.

## **Why the Correct Answer is Right**
The correct answer, **C. Cytotoxic T lymphocytes (CTLs) or CD8+ T cells**, are crucial for their ability to recognize and kill infected cells or tumor cells. They do so by recognizing antigens presented on the surface of these cells via MHC class I molecules. Upon activation, CTLs release cytotoxins that induce apoptosis in target cells, thereby controlling viral infections and eliminating cancer cells.

## **Why Each Wrong Option is Incorrect**
- **Option A: Neutrophils** are primarily involved in the defense against bacterial infections and are not the key players in combating viruses and cancer cells directly.
- **Option B: Helper T cells (CD4+ T cells)** play a vital role in coordinating the immune response, including activating B cells and cytotoxic T cells, but they do not directly kill infected cells or cancer cells.
- **Option D: Basophils** are mainly involved in inflammatory reactions, particularly in allergy and anaphylaxis, and are not directly involved in the fight against viruses and cancer.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Cytotoxic T lymphocytes (CTLs)** are essential for cell-mediated immunity against viral infections and tumor surveillance. Their dysfunction can lead to severe immunodeficiency and increased susceptibility to viral infections and cancer. CTLs can be harnessed in cancer therapy, such as in the use of checkpoint inhibitors to enhance their activity against tumor cells.

## **Correct Answer:** C. Cytotoxic T lymphocytes.