The correct answer is A, septum. It derives from the Latin term, "saeptum", which means dividing a wall or enclsoure. It is a hole in wall which seperates the chambers of the heart. There are various septums in the heart. The atrial septum seperates the two upper chambers which are the left and right atria.
The ventricular septum seperates the two lower chambers, which are the left and right ventricles.There is also an atrioventrical septum which seperates the left ventricle from the right atria. All of the septums work together to keep carbon-clioxide rich and oxygen-poor from entering the pulmonary veins.