**Core Concept**
In the context of head injuries, hemorrhage refers to bleeding within the brain or surrounding structures. The type and location of hemorrhage are crucial in determining the severity and outcome of the head injury.

**Why the Correct Answer is Right**
The commonest hemorrhage following head injury is typically an epidural hematoma (EDH). This type of hemorrhage occurs when there is a tear in the middle meningeal artery, which is located between the dura mater and the inner surface of the skull. The blood accumulates in the epidural space, leading to increased intracranial pressure and potential brain herniation. EDH is often associated with a lucid interval, where the patient may appear to recover initially before deteriorating due to the expanding hematoma.

**Why Each Wrong Option is Incorrect**
* **Option A:** Subdural hematoma (SDH) is another type of hemorrhage that can occur after head injury, but it is less common than EDH. SDH occurs when there is a tear in the bridging veins between the brain and the dura mater.
* **Option B:** Subarachnoid hemorrhage (SAH) is a type of hemorrhage that occurs in the space between the brain and the pia mater. While it can be a life-threatening condition, it is not the most common type of hemorrhage following head injury.
* **Option D:** Cerebral contusion refers to bruising of the brain tissue, which can occur after a head injury. While it can be a serious condition, it is not typically classified as a hemorrhage.

**Clinical Pearl / High-Yield Fact**
In the context of head injuries, the "lucid interval" refers to a period of time where the patient appears to recover or show improvement before deteriorating due to an expanding epidural hematoma. This is a critical concept to recognize, as it can indicate the need for immediate surgical intervention.

**Correct Answer: C. Epidural hematoma.**