Answer:
The correct answer would be C. The valve is between the atria in the ventricles.
The first part of the heartbeat includes the flow of blood from atria to their respective ventricles.
During this, atria contracts due to which atrioventricular valves (mitral and tricuspid valves) are open and blood flow down to the ventricles.
Thus, if a doctor finds fluttering in the first part of the heartbeat, then there must be a problem associated with the valve present between atria and ventricles that is atrioventricular valves.