**Core Concept**
Wernicke's encephalopathy is a neurological disorder caused by **thiamine (Vitamin B1) deficiency**, characterized by a classical triad of symptoms: confusion, ophthalmoplegia, and ataxia. The condition is often associated with chronic alcoholism but can also result from other causes of malnutrition.

**Why the Correct Answer is Right**
The earliest symptom to show improvement with thiamine therapy is typically **ophthalmoplegia**, as it is the most responsive to thiamine replacement. Thiamine plays a critical role in **energy metabolism** and **nerve function**, and its deficiency leads to the dysfunction of the extraocular muscles, resulting in ophthalmoplegia.

**Why Each Wrong Option is Incorrect**
**Option A:** Confusion is a symptom of Wernicke's encephalopathy but is not the first to improve with thiamine therapy.
**Option B:** Ataxia, another component of the triad, also improves but not as promptly as ophthalmoplegia.
**Option D:** This option is not provided, but typically, other symptoms or signs not part of the classical triad are less relevant to the immediate response to thiamine.

**Clinical Pearl / High-Yield Fact**
It is crucial to recognize Wernicke's encephalopathy early and treat it promptly with thiamine to prevent progression to Korsakoff syndrome, a more severe and often irreversible condition.

**Correct Answer:** C. Ophthalmoplegia.