Final answer:
The 'dub' sound of the second heart sound is caused by the closing of the semilunar valves (option b), preventing backflow of blood into the ventricles during ventricular diastole.
Step-by-step explanation:
The second heart sound, referred to as S2 or "dub," is due to the closing of the semilunar valves during ventricular diastole. When the ventricles relax after contraction, the pressure within them falls below the pressure in the aorta and pulmonary artery, causing the semilunar valves to snap shut. This closure prevents backflow of blood into the ventricles and is associated with a sound caused by the turbulence of blood flow, which we hear as the "dub" sound in the heartbeat pattern 'lub-dub.'