**Core Concept**
The condition described is a type of intracranial hemorrhage characterized by the accumulation of fluid in the subdural space, leading to compression of the underlying brain tissue. This occurs due to the rupture of bridging veins that connect the cerebral cortex to the dural sinuses.

**Why the Correct Answer is Right**
The subdural hematoma is caused by the tearing of bridging veins, which are susceptible to injury due to their thin walls and location between the cerebral cortex and the dura mater. As a result, blood accumulates in the subdural space, compressing the brain and leading to increased intracranial pressure. The patient's symptoms, including somnolence, are a result of the compression of the left cerebral hemisphere.

**Why Each Wrong Option is Incorrect**
**Option A:** Epidural hematoma typically occurs due to the rupture of the middle meningeal artery, which is located between the dura mater and the inner table of the skull. This condition often presents with a more rapid onset of symptoms and a characteristic "lucid interval" before deterioration.
**Option B:** Subarachnoid hemorrhage refers to bleeding into the space between the arachnoid mater and the pia mater, which surrounds the brain. This condition is often associated with the rupture of an aneurysm or arteriovenous malformation.
**Option C:** Cerebral edema is a condition characterized by the accumulation of fluid in the brain tissue itself, often due to inflammation, infection, or injury. While it can lead to increased intracranial pressure, it is distinct from a subdural hematoma.

**Clinical Pearl / High-Yield Fact**
The patient's age and history of trauma increase the likelihood of a subdural hematoma, as the bridging veins are more susceptible to injury in older adults. It is essential to consider this diagnosis in patients with a history of head trauma, especially if they present with symptoms of increased intracranial pressure.

**Correct Answer: C. Subdural hematoma**