**Core Concept**
During inspiration, the diaphragm contracts and descends, increasing the volume of the thoracic cavity. This increase in volume leads to a decrease in intrapleural pressure, allowing air to enter the lungs.

**Why the Correct Answer is Right**
As the diaphragm contracts, it separates the abdominal contents from the thoracic cavity, creating a pressure gradient that favors the entry of air into the lungs. The decrease in intrapleural pressure is due to the increased volume of the thoracic cavity, which stretches the lungs and the pleural membranes, reducing the intrapleural pressure. This decrease in intrapleural pressure allows the alveoli to expand and draw in air.

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable (intrapleural pressure change not specified).
**Option B:** The intrapleural pressure does not become positive during inspiration. A positive intrapleural pressure would push the lungs out of the thoracic cavity.
**Option C:** The intrapleural pressure does not become zero during inspiration. A zero intrapleural pressure would mean that the lungs are not expanding, and no air would enter the lungs.

**Clinical Pearl / High-Yield Fact**
Remember that the diaphragm is the primary muscle responsible for inspiration, and its contraction leads to a decrease in intrapleural pressure, allowing air to enter the lungs.

**Correct Answer: D. Decreases**