From which of the following does the right atrium receive blood?
a: pulmonary arteries
b: pulmonary veins
c: aorta
d: superior vena cava

The right atrium receives blood returning from others parts of the body through the superior vena cava and the inferior vena cava.  

Explanation:

The pathway of circulation begins in the right atrium which receives the carbon dioxide-rich deoxygenated blood returning through the systemic circulation.  

The deoxygenated blood from regions superior to the heart, i.e., the head, neck, shoulder areas are collected through the superior vena cava and that from the parts inferior or lower to the heart like visceral organs, extremities, trunk, hip etc are brought through the inferior vena cava.

Both these venous systems (superior and inferior) fill the right atrium.

The right atrium then pumps the deoxygenated blood to the right ventricle via the tricuspid valve.

The right atrium is filled with blood during diastole.