Which of the following contributes to the formation of the vena cava, coronary sinus and posterior walls of the right atrium and left atrium?

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The key idea is how the venous structures in the early heart remodel to form the atrial walls and the main venous openings. The sinus venosus is the primitive venous reservoir that, as the heart develops, gets incorporated into the atria. The right horn becomes the smooth posterior wall of the right atrium (the sinus venarum) and provides the openings for the superior and inferior vena cavae. The left horn becomes the coronary sinus, which drains into the right atrium. Because of these transformations, the sinus venosus directly contributes to the vena cava openings, the coronary sinus, and the posterior portion of the right atrium. The other primitive components contribute to different regions: the atrioventricular canal forms the AV junctions and valves, the bulbus cordis contributes to the outflow tract of the heart, and the truncus arteriosus forms the great vessels.