**Core Concept**
The identification of arterial injuries is crucial in trauma cases, and various signs can be used to diagnose these injuries. Hard signs of arterial injury are those that are immediately life-threatening and require prompt intervention. These signs include active bleeding, a pulse deficit, a thrill or bruit over the affected artery, and a hard or expanding hematoma.

**Why the Correct Answer is Right**
The correct answer is a hard sign that requires immediate attention. A hard or expanding hematoma is a sign of arterial injury due to the high pressure and flow of blood from the arterial system. This can lead to further complications such as compartment syndrome or exsanguination if not treated promptly.

**Why Each Wrong Option is Incorrect**
**Option A:** Active bleeding is a hard sign of arterial injury, as it indicates a significant loss of blood and can lead to hypovolemic shock.
**Option B:** A thrill or bruit over the affected artery is a hard sign of arterial injury, as it indicates turbulent blood flow and a possible arterial disruption.
**Option C:** A hard or expanding hematoma is a hard sign of arterial injury, as it indicates the accumulation of blood under pressure from the arterial system.

**Clinical Pearl / High-Yield Fact**
It is essential to identify hard signs of arterial injury promptly, as they can lead to life-threatening complications. A high index of suspicion and a thorough physical examination are crucial in diagnosing arterial injuries.

**Correct Answer: D. None of the above.**