**Core Concept**
Surfactant is a complex mixture of phospholipids and proteins produced by type II alveolar cells in the lungs. It plays a crucial role in maintaining the structural and functional integrity of the alveolar surface.

**Why the Correct Answer is Right**
The primary function of surfactant is to reduce the surface tension within the alveoli, thereby preventing lung collapse and facilitating gas exchange. Surfactant achieves this by coating the alveolar surface with a thin layer of phospholipids, which decreases the interfacial tension between the air and the alveolar fluid. This reduction in surface tension allows the alveoli to remain open and expand during inhalation, improving lung compliance and gas exchange efficiency. The surfactant protein SP-B is particularly important in this process, as it helps to stabilize the surfactant film and prevent its collapse.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while surfactant does play a role in preventing lung collapse, its primary function is not related to the clearance of alveolar fluid.

**Option B:** This option is incorrect because surfactant is not primarily involved in the regulation of lung blood flow.

**Option C:** This option is incorrect because surfactant is not primarily involved in the maintenance of the alveolar-capillary barrier.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that surfactant deficiency or dysfunction can lead to respiratory distress syndrome (RDS) in premature infants, characterized by lung collapse and impaired gas exchange.

**Correct Answer: D. Reducing surface tension within the alveoli.**