The correct answer is (a) S. A. Nodes.
The S. A. Nodes, or sinoatrial node, is a small group of cells located in the upper right chamber of the heart (the right atrium). It is the heart’s natural pacemaker, meaning that it generates the electrical signals that cause the heart to beat.
The A. V. Nodes, or atrioventricular node, is a group of cells located between the upper and lower chambers of the heart (the atria and ventricles). It receives electrical signals from the S. A. Nodes and then passes them on to the Bundle of His, which is a bundle of fibers that carries the electrical signals to the ventricles.
The Chordaetendinal are small, cord-like structures that attach the mitral and tricuspid valves to the papillary muscles of the heart. They help to prevent the valves from prolapsing (flipping back) into the atria when the ventricles contract.
In summary, the S. A. Nodes are the heart’s natural pacemaker, while the A. V. Nodes, Bundle of His, and Chordaetendinal are all important structures that help to regulate the heart’s rhythm.