Question

how does blood move back to the heart?
the muscles in veins and other parts of the body help move blood back to the heart.
the muscles in arteries and other parts of the body help move blood back to the heart.
blood naturally flows back toward the heart, thanks to gravity.
the muscles of the heart pump the blood back toward itself

Explanation:

Veins carry blood back to the heart. The muscles in veins (venous muscles) and skeletal - muscle contractions in other parts of the body help push blood against gravity back to the heart. Arteries carry blood away from the heart. Gravity alone is not sufficient to move blood back to the heart, especially from lower parts of the body. The heart muscles pump blood out of the heart, not back into it in the context of returning blood from the body.

Answer:

The muscles in veins and other parts of the body help move blood back to the heart.