**Core Concept**
Surfactant is a complex substance produced in the lungs that reduces surface tension within the alveoli, facilitating easier expansion during inhalation and preventing alveolar collapse during exhalation. This is crucial for maintaining normal respiratory function.

**Why the Correct Answer is Right**
Surfactant is composed of lipids and proteins, specifically phospholipids and surfactant-associated proteins (SP-A, SP-B, SP-C, and SP-D). These components interact to reduce the surface tension at the air-liquid interface within the alveoli. The primary surfactant protein, SP-B, plays a crucial role in surfactant function by facilitating the assembly of phospholipids into stable, surface-active films. This results in a significant reduction in surface tension, making it easier to expand the alveoli during inhalation.

**Why Each Wrong Option is Incorrect**
**Option A:** Sugar and salt are not components of surfactant; they play other crucial roles in the body, such as maintaining osmotic balance and facilitating glucose and sodium transport.
**Option B:** Soap and water are not the primary components of surfactant; soap is a detergent used for cleaning, and water is a solvent.
**Option D:** Base and lipid do not accurately describe the composition of surfactant; the primary components are phospholipids and surfactant-associated proteins.

**Clinical Pearl / High-Yield Fact**
Patients with surfactant deficiency, such as those with respiratory distress syndrome (RDS), often require treatment with exogenous surfactant to reduce surface tension and improve lung function.

**✓ Correct Answer: C. Lipid and protein**