The "dub" is the second sound (S2) and marks the end of systole and the beginning of , the phase when the heart relaxes and refills with blood.
If the statement is presented as a or multiple-choice question, the correct completion is: the lub dub sound the heart makes is caused by
the closure of the atrioventricular (AV) valves — specifically the tricuspid and mitral valves — at the beginning of ventricular systole (contraction). The "dub" (second heart sound, S2) is caused by the closure of the semilunar valves — the aortic and pulmonary valves — at the beginning of ventricular diastole (relaxation). The "dub" is the second sound (S2) and
"...the closure of the heart valves."