The correct option is A Chordae tendinae
The chordae tendinae are strong, fibrous strings attached to the leaflets (or cusps) of the heart on the ventricular side i.e., the lower chamber. These strings originate from small mounds of muscle tissue, the papillary muscles, which project inward from the walls of the ventricle. When the cusps close, the chordae tendinae prevent them from swinging back into the atrium cavity (the upper chamber).