The correct answer is d. AV valves close.
The atrioventricular valves (AV valves), separate the atria from the ventricles and allow one-way blood flow. The right AV valves is a tricuspid valve and the left AV valve is a mitral valve. When the atrial pressure exceeds ventricular pressure, the AV valves are opened and blood flows into the ventricles. On the other hand, when the ventricular pressure exceeds atrial pressure the AV valves are closed.