**Core Concept**
Pulmonary surfactant is a complex mixture of phospholipids and proteins that reduces surface tension in the lungs, preventing alveolar collapse and facilitating gas exchange. Its secretion is crucial for maintaining normal lung function, particularly during expiration.

**Why the Correct Answer is Right**
The correct answer is Type II pneumocytes. These cells are responsible for producing and secreting pulmonary surfactant, which is then distributed throughout the alveoli. The surfactant is composed of phospholipids, such as dipalmitoylphosphatidylcholine (DPPC), and proteins, including surfactant protein A (SP-A), SP-B, SP-C, and SP-D. The surfactant proteins play a vital role in surfactant function, including film formation, stability, and clearance.

**Why Each Wrong Option is Incorrect**
**Option A:** Type I pneumocytes are not responsible for surfactant production. They are flat, squamous cells that form the majority of the alveolar surface area and are involved in gas exchange.

**Option B:** Clara cells are non-ciliated, secretory cells found in the bronchioles and are involved in the production of surfactant protein D (SP-D) and other substances, but not the main surfactant component.

**Option C:** Alveolar macrophages are immune cells that reside in the alveoli and are involved in the clearance of pathogens and debris, but not in surfactant production.

**Clinical Pearl / High-Yield Fact**
Pulmonary surfactant deficiency or dysfunction can lead to respiratory distress syndrome (RDS) in premature infants and acute respiratory distress syndrome (ARDS) in adults. Understanding the role of Type II pneumocytes in surfactant production is crucial for managing these conditions.

**Correct Answer:** D. Type II pneumocytes.