Final answer:
The classic M-mode finding for atrial septal defect is right ventricular volume overload.
Step-by-step explanation:
The classic M-mode finding for atrial septal defect is the B. Right ventricular volume overload. In an atrial septal defect, there is a hole in the atrial septum which allows increased blood flow from the left atrium to the right atrium. This leads to increased volume in the right ventricle as it pumps blood to the lungs. This volume overload in the right ventricle is the characteristic finding in M-mode echocardiography for atrial septal defect.