Final answer:
Contrast echocardiography is the best method to visualize shunt flow across an atrial septal defect, providing clearer images of blood flow through the use of a contrast agent.
Step-by-step explanation:
The method that would best allow visualization of shunt flow across an atrial septal defect would be C. Contrast echocardiography.
This technique is ideal for assessing blood flow across heart structures where there's an abnormal communication, such as an atrial septal defect, because it uses ultrasound combined with a contrast agent that provides a clearer image of the blood flow patterns inside the heart.
Contrast echocardiography can reveal the presence of the shunted blood, which is not visibly distinct in standard echocardiography. Options A, B, and D (M-mode echocardiography, Two-dimensional echocardiography, Stress echocardiography) are also important in cardiac assessment but are not as effective as contrast echocardiography in visualizing the flow of blood, specifically in the case of an atrial septal defect shunt.