**Core Concept**
The question tests the understanding of the mechanisms involved in the killing of intracellular bacteria. Intrinsic immunity, which includes the action of certain cellular components, plays a crucial role in the defense against intracellular pathogens. The correct answer relates to the production of reactive oxygen species (ROS) and the activation of a specific enzyme.

**Why the Correct Answer is Right**
The correct answer involves the activation of the enzyme NADPH oxidase, which is a key component of the respiratory burst in phagocytic cells. This enzyme is crucial for the production of ROS, such as superoxides, which are toxic to intracellular bacteria. The activation of NADPH oxidase is a critical step in the killing of intracellular pathogens, including bacteria like Listeria monocytogenes and Mycobacterium tuberculosis. The production of ROS leads to the damage of the bacterial cell membrane and ultimately leads to the death of the bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly relate to the killing of intracellular bacteria. While complement proteins can opsonize bacteria, they do not directly contribute to their killing within host cells.

**Option B:** This option is incorrect because it is related to the extracellular killing of bacteria, not intracellular bacteria. The action of neutrophil defensins is crucial for the killing of extracellular bacteria.

**Option C:** This option is incorrect because it refers to a mechanism of killing extracellular bacteria, not intracellular bacteria. The action of eosinophils is primarily involved in the killing of parasites and in the pathogenesis of allergic reactions.

**Clinical Pearl / High-Yield Fact**
The killing of intracellular bacteria requires the coordinated action of several cellular components, including the activation of NADPH oxidase and the production of ROS. This process is critical for the defense against pathogens that can replicate within host cells, such as Listeria monocytogenes and Mycobacterium tuberculosis.

**Correct Answer:** C. NADPH oxidase.