**Core Concept**
The question tests the understanding of the physiological factors influencing airway resistance during breathing. Airway resistance is the opposition to airflow in the airways, and it is affected by various factors such as airway diameter, airway length, and airway wall properties.

**Why the Correct Answer is Right**
During expiration, the airways are compressed more than during inspiration due to the greater outward recoil of the lungs and the increased pressure in the thoracic cavity. This compression of the airways increases their internal diameter, leading to a decrease in airway resistance during inspiration. However, during expiration, the airways are compressed even more, increasing their internal diameter and resistance. The increased compression of the airway is the primary reason for more resistance in expiration. The airway's elastic recoil and the pressure gradient between the lungs and the atmosphere contribute to this phenomenon.

**Why Each Wrong Option is Incorrect**
**Option B:** The change from linear to turbulent flow does not significantly affect airway resistance during expiration. Turbulent flow may occur at higher flow rates, but it is not the primary reason for increased resistance during expiration.

**Option C:** Saturation with moisture does not directly affect airway resistance during expiration. While excess mucus can increase airway resistance, it is not the primary factor in this context.

**Option D:** An increased rate of flow during expiration does not directly cause more resistance in expiration. In fact, increased flow rates are often associated with decreased resistance.

**Clinical Pearl / High-Yield Fact**
The increased compression of the airway during expiration is a critical factor in determining airway resistance, and it is essential to consider this when evaluating respiratory function in patients.

**✓ Correct Answer: A. Increased compression of airway**