**Core Concept**
The basal ganglia are a group of subcortical structures involved in movement control and cognition. The nucleus of the basal ganglia is a critical component that plays a key role in the regulation of motor movements. The main nuclei include the caudate nucleus, putamen, globus pallidus, substantia nigra, and subthalamic nucleus.

**Why the Correct Answer is Right**
The correct answer is the globus pallidus, which is one of the main nuclei of the basal ganglia. The globus pallidus is divided into two main segments: the internal globus pallidus (GPi) and the external globus pallidus (GPe). The GPi is the primary output of the basal ganglia, and it is involved in the regulation of motor movements by projecting to the thalamus and the brainstem. The GPe, on the other hand, is involved in the regulation of the GPi and the substantia nigra.

**Why Each Wrong Option is Incorrect**
**Option A:** The caudate nucleus is another main nucleus of the basal ganglia, but it is not the correct answer. The caudate nucleus is involved in the regulation of movement and cognition, but it is not the primary output of the basal ganglia.
**Option B:** The putamen is a main nucleus of the basal ganglia, but it is not the correct answer. The putamen is involved in the regulation of movement and cognition, but it is not the primary output of the basal ganglia.
**Option D:** The substantia nigra is a main nucleus of the basal ganglia, but it is not the correct answer. The substantia nigra is involved in the regulation of movement and cognition, but it is not the primary output of the basal ganglia.

**Clinical Pearl / High-Yield Fact**
The basal ganglia are often affected in movement disorders such as Parkinson's disease and Huntington's disease. The globus pallidus is a key structure in the pathophysiology of these disorders, and it is often targeted for deep brain stimulation.

**Correct Answer: C. Globus pallidus.**