**Core Concept**
Seizures in a premature neonate can be caused by various factors, including hypoxic-ischemic encephalopathy, intraventricular hemorrhage, and electrolyte imbalances. In this scenario, the investigation of choice is one that helps differentiate between these potential causes.

**Why the Correct Answer is Right**
In a premature neonate presenting with generalized tonic-clonic seizures (GTCS), a crucial investigation is the **non-contrast cranial ultrasound**. This imaging modality is particularly useful in this age group as it is non-invasive and does not involve radiation exposure. Ultrasound can help identify intraventricular hemorrhages, periventricular leukomalacia, and other structural abnormalities that may be causing the seizures.

**Why Each Wrong Option is Incorrect**
**Option A:** Computed Tomography (CT) scan is not typically used in the first few days of life due to radiation concerns and the fact that CT scans may not provide sufficient detail in this age group.
**Option B:** Magnetic Resonance Imaging (MRI) is not usually the first-line investigation in this scenario, although it may be used later to provide more detailed information about the brain.
**Option C:** Electroencephalogram (EEG) can help diagnose seizures and identify the type of seizure disorder, but it does not provide information about the underlying cause of the seizures.

**Clinical Pearl / High-Yield Fact**
In premature neonates, seizures can be a sign of underlying brain injury, and prompt investigation and treatment are crucial to prevent further brain damage and improve outcomes.

**Correct Answer: C. EEG**