**Core Concept**
The question is testing the ability to identify a characteristic radiological feature of an Atrial Septal Defect (ASD). An ASD is a congenital heart defect characterized by an abnormal opening in the atrial septum, allowing blood to flow between the left and right atria.

**Why the Correct Answer is Right**
The characteristic X-ray finding in ASD is an enlarged cardiac silhouette, often referred to as cardiomegaly. This is due to the increased volume of blood flowing through the heart, leading to dilation of the cardiac chambers. Additionally, the increased flow through the pulmonary vessels can cause prominent pulmonary vascular markings on the X-ray. The increased flow through the heart also leads to increased pressure in the right atrium and ventricle, resulting in right atrial enlargement.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is not a characteristic X-ray finding in ASD. A normal cardiac silhouette is expected in ASD.
* **Option B:** This is a characteristic X-ray finding in Tetralogy of Fallot, not ASD. Tetralogy of Fallot is characterized by right ventricular outflow tract obstruction, which leads to decreased pulmonary vascular markings.
* **Option D:** This is a characteristic X-ray finding in Pulmonary Hypertension, not ASD. Pulmonary Hypertension is characterized by increased pressure in the pulmonary arteries, leading to small pulmonary vascular markings.

**Clinical Pearl / High-Yield Fact**
Remember that the characteristic X-ray finding in ASD is an enlarged cardiac silhouette, often with prominent pulmonary vascular markings.

**Correct Answer: C. Enlarged cardiac silhouette.**