**Core Concept**
The process of white blood cells (WBCs) crossing the endothelium, a layer of flat cells lining blood vessels, is a critical step in the inflammatory response. This process involves the movement of WBCs through the endothelial layer, allowing them to migrate towards sites of infection or injury.

**Why the Correct Answer is Right**
Diapedesis is the specific term used to describe the transmigration of WBCs across the endothelium. This process involves the activation of adhesion molecules on the surface of endothelial cells, which bind to integrins on the surface of WBCs, facilitating their movement through the endothelial layer. The endothelial cells also undergo changes in their cytoskeleton, creating openings or gaps that allow WBCs to pass through. This process is an essential component of the inflammatory response, enabling WBCs to reach sites of infection or injury.

**Why Each Wrong Option is Incorrect**
**Option A:** Margination refers to the process by which WBCs slow down and come to rest against the endothelial wall, preparing for the next step in the inflammatory response. It is an important step but does not involve the actual crossing of the endothelium.

**Option B:** Rolling is the initial step in the inflammatory response, where WBCs slow down and start to move along the endothelial surface due to the interaction with selectins. While rolling is an essential step, it does not involve the actual transmigration of WBCs across the endothelium.

**Option C:** Pavementing is a term used to describe the tight packing of WBCs against the endothelial wall, often seen in conditions such as vasculitis. While pavementing is an interesting phenomenon, it is not the same as diapedesis.

**Clinical Pearl / High-Yield Fact**
Diapedesis is a critical step in the inflammatory response, and its dysregulation can lead to various pathological conditions, including sepsis and autoimmune diseases. Understanding the mechanisms of diapedesis is essential for the diagnosis and management of these conditions.

**✓ Correct Answer: D. Diapedesis**