**Core Concept**
The alveoli are tiny air sacs in the lungs where gas exchange occurs. To facilitate efficient gas exchange, the alveoli must remain hydrated and free of fluid. The mechanism to prevent alveolar flooding is crucial for maintaining normal respiratory function.

**Why the Correct Answer is Right**
The correct mechanism to keep alveoli dry is the surfactant system. Surfactant is a complex mixture of phospholipids and proteins produced by type II pneumocytes. It reduces surface tension in the alveoli, preventing them from collapsing and flooding with fluid. Additionally, surfactant helps to stabilize the alveolar structure and facilitates gas exchange.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the alveolar macrophages primarily function in clearing dust and debris from the alveoli, rather than maintaining their hydration.

**Option B:** This option is incorrect because the terminal bronchioles are involved in the conduction of air, but they do not directly contribute to maintaining alveolar hydration.

**Option C:** This option is incorrect because the Clara cells, also known as Club cells, are involved in the production of surfactant and other substances, but they do not directly keep alveoli dry.

**Clinical Pearl / High-Yield Fact**
Surfactant deficiency is a key feature of respiratory distress syndrome (RDS) in preterm infants, leading to alveolar collapse and impaired gas exchange.

**Correct Answer:** C. Surfactant.