**Core Concept**
The patient's symptoms suggest a condition involving a sudden, severe headache accompanied by neurological deficits, specifically cranial nerve palsy. This presentation is characteristic of a non-atherosclerotic, non-traumatic cause of subarachnoid hemorrhage, often due to the rupture of an aneurysm.

**Why the Correct Answer is Right**
The patient's thunderclap headache, which is a severe headache that reaches its maximum intensity within a minute, is suggestive of subarachnoid hemorrhage. The progressive III cranial nerve palsy, which involves the oculomotor nerve, can occur due to compression or stretching of the nerve by a blood clot or aneurysm in the subarachnoid space. The sudden onset of unconsciousness suggests a severe increase in intracranial pressure.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the patient's symptoms. The symptoms described do not suggest a diagnosis of meningitis, which typically presents with fever, headache, and stiff neck.
* **Option B:** This option is not a likely diagnosis. While a posterior communicating artery aneurysm can cause III cranial nerve palsy, the sudden onset of unconsciousness and thunderclap headache suggest a more severe condition.
* **Option C:** This option is not relevant to the patient's symptoms. The symptoms described do not suggest a diagnosis of temporal arteritis, which typically presents with headache, scalp tenderness, and jaw claudication.

**Clinical Pearl / High-Yield Fact**
A thunderclap headache is a medical emergency and should prompt an immediate search for a subarachnoid hemorrhage. The presence of III cranial nerve palsy suggests a specific cause of subarachnoid hemorrhage, such as a posterior communicating artery aneurysm.

**Correct Answer: C. Posterior communicating artery aneurysm.**