Final answer:
The tricuspid valve is closed when the ventricles are in systole, as the pressure rises within the ventricles causing blood to flow towards the area of lowest pressure initially in the atria.
Step-by-step explanation:
The tricuspid valve is closed when the ventricles are in systole, option D. When the ventricles contract during systole, pressure within the ventricles rises, causing blood to flow towards the area of lowest pressure initially in the atria. This backflow of blood closes the tricuspid valve to prevent blood from being forced back into the atria.