**Core Concept**
Brain death is a clinical diagnosis characterized by the irreversible loss of all functions of the brain, including the brainstem. This condition is diagnosed using a combination of clinical and laboratory tests, and it is essential to identify the features that distinguish brain death from other neurological conditions.

**Why the Correct Answer is Right**
Brain death is associated with the loss of all cranial nerve reflexes, including the oculo-vestibular reflex and the corneal reflex. The absence of these reflexes is a critical component of the diagnosis of brain death. In contrast, pain perception is preserved in brain death, as the spinal cord and peripheral nervous system are not affected. The preservation of pain perception is a key feature that distinguishes brain death from other conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** Doll's eye movement is actually a sign of brain death, as it indicates the absence of oculoccephalic reflexes, which are mediated by the brainstem.
**Option B:** The oculo-vestibular reflex is indeed absent in brain death, as it is mediated by the brainstem and is a critical component of the diagnosis.
**Option D:** The corneal reflex is also absent in brain death, as it is mediated by the brainstem and is a key feature of brain death.

**Clinical Pearl / High-Yield Fact**
It is essential to note that brain death is a clinical diagnosis, and the absence of certain reflexes and responses is critical in making this diagnosis. The preservation of pain perception in brain death is a key feature that distinguishes it from other conditions.

**✓ Correct Answer: C. Only pain is preserved**