**Core Concept**
Subarachnoid hemorrhage (SAH) is a life-threatening condition characterized by bleeding into the space between the brain and the tissue that covers it, leading to increased intracranial pressure and potential brain damage. The most common cause of SAH is the rupture of a vascular structure in the brain.

**Why the Correct Answer is Right**
The correct answer, **C. Berry aneurysm**, is the most common cause of spontaneous subarachnoid hemorrhage. Berry aneurysms are small, saccular aneurysms that occur at the junctions of major arteries in the Circle of Willis, a critical area at the base of the brain. These aneurysms are prone to rupture due to their thin walls and the high pressure of the surrounding blood vessels. The rupture of a berry aneurysm leads to bleeding into the subarachnoid space, causing SAH.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypertension is a risk factor for SAH, but it is not the most common cause. While chronic hypertension can lead to aneurysm formation and rupture, it is not the primary underlying cause.
**Option B:** Arteriovenous malformations (AVMs) can cause SAH, but they are a less common cause compared to berry aneurysms. AVMs are abnormal connections between arteries and veins that can lead to bleeding, but they are typically more associated with intracerebral hemorrhage.
**Option D:** Tumors can cause SAH, but they are a rare cause compared to berry aneurysms. Tumors can compress or erode blood vessels, leading to SAH, but this is typically seen in specific contexts, such as metastatic disease.

**Clinical Pearl / High-Yield Fact**
Berry aneurysms are often asymptomatic until rupture, which is why they are often referred to as "silent killers." Early detection and treatment of berry aneurysms are critical to prevent SAH.

**✓ Correct Answer: C. Berry aneurysm**