Sinus venosus is triangular chamber present on the dorsal side of heart and opens into right auricle. In mammals, sinus venosus exists distinctly only in the embryonic heart. Sinus venosus is not functional in mammal's heart. The heart of rabbit is four-chambered with two auricles and two ventricles and the Sinus venosus is absent as it is fused into right auricle. So, the correct answer is option C.