**Core Concept**
Antitumor activity refers to the immune system's ability to recognize and eliminate cancer cells. This involves various immune cells that can directly or indirectly kill tumor cells.

**Why the Correct Answer is Right**
Basophils are primarily involved in inflammation and allergic reactions, not antitumor activity. They release histamine and other mediators that promote inflammation, which can sometimes inadvertently damage tumor cells but are not a primary mechanism of antitumor defense. In contrast, **cytotoxic T lymphocytes** (CTLs) and **natural killer cells** play crucial roles in antitumor immunity. CTLs directly kill tumor cells by recognizing and binding to specific antigens, while natural killer cells recognize and destroy cancer cells without prior antigen exposure. **Macrophages**, particularly tumor-associated macrophages, can also contribute to antitumor activity by engulfing and digesting tumor cells or by producing cytokines that activate other immune cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Cytotoxic T lymphocytes are a key component of antitumor immunity.
**Option B:** Natural killer cells are another crucial player in recognizing and eliminating tumor cells.
**Option C:** Basophils are not directly involved in antitumor activity.
**Option D:** Macrophages can contribute to antitumor activity, particularly in the tumor microenvironment.

**Clinical Pearl / High-Yield Fact**
The immune system's ability to recognize and eliminate cancer cells is a complex process involving multiple cell types, including CTLs, natural killer cells, and macrophages. Understanding the role of these immune cells in antitumor activity is essential for developing effective cancer therapies.

**✓ Correct Answer: C. Basophils**