The correct option is A Dorsal surface of the heart of frog
The thin-walled sac in the centre of the dorsal surface is the sinus venous. It is formed by the union of three veins as the single post-caval vein or caudal vena cava and the right and left precaval veins or cranial vena cavae. The sinus venous transports deoxygenated blood into the right atrium.It also has cardiac muscles that act as pacemaker for the heart. Adult mammals do not possess a sinus venosus.
So, the correct answer is option A.