What are the 5 mechanisms that keep venous blood moving towards the heart?

I think you meant toward the right atrium, not the heart. Here are the 5 mechanisms:

1. Continued beating of the heart, which pumps blood through the entire circulatory system.

2. Adequate blood pressure in the arteries, to push blood to and through the veins.

3. Semilunar valves in the veins that ensure continued blood flow in one direction (toward the heart).

4. Contraction of skeletal muscles, which squeezes veins, producing a kind of pumping action.

5. Changing pressures in the chest cavity during breathing that produce a kind of pumping action in the veins in the thorax.

The correct answer would be b