The correct option is B The sinus venosus
The heart of a human has two atria, a mitral valve, and four chambers but does not have sinus venosus. Sinus venosus is a thin-walled, enlarged region between the vena cava and the right atrium. It is the pacemaker in a frog.
In mammals, it exists distinctly only during embryonic development (where it is positioned between the two vena cava); in non-mammalian chordates, the sinus venosus persists into the adult stages.
The mitral valve (also known as the bicuspid valve or left atrioventricular valve) is a double-flap valve. In the heart that lies between the left atrium (LA) and the left ventricle (LV).
So, the correct answer is option C.