**Core Concept:** Ataxia, nystagmus, absent deep tendon reflexes, and extensor plantar response are features of cerebellar dysfunction. The cerebellum plays a crucial role in maintaining balance, coordination, and reflexes.

**Why the Correct Answer is Right:** The correct answer is C: **Spinocerebellar degeneration (SCD)**. Spinocerebellar degenerations are a group of inherited neurodegenerative disorders that affect the cerebellum and spinal cord. In SCD, the cerebellum degenerates, leading to the clinical manifestations described in the question, such as ataxia, nystagmus, absent deep tendon reflexes, and extensor plantar response.

**Why Each Wrong Option is Incorrect:**

A. **Huntington's disease (HD)** involves both basal ganglia and cerebellum, leading to more complex motor symptoms (chorea, dystonia, bradykinesia), cognitive impairment, and psychiatric symptoms.
B. **Wernicke-Korsakoff syndrome** is a result of thiamine deficiency and affects the brainstem and thalamus, leading to Korsakoff's psychosis and Wernicke's encephalopathy.
D. **Multiple Sclerosis (MS)** typically presents with a combination of sensory, motor, and cerebellar symptoms but does not typically cause nystagmus and extensor plantar response.

**Clinical Pearl:** The presence of extensor plantar response is a classic feature of spinocerebellar degenerations, which helps to differentiate it from other neurological conditions presenting with cerebellar signs.

**Correct Answer:** C. Spinocerebellar degeneration (SCD) is the most likely diagnosis in this case due to the specific clinical findings described in the question.