Correct answer: B) H+ ions flow out of the thylakoid.
It is the flowing of H+ ions through the ATP synthase from inside of the thylakoid to the outside of the thylakoid that allows for the phosphate group to be added to ADP molecules, forming ATP molecules by the ATP synthase. This flow of H+ ions is promoted by the increase of H+ ions inside of the thylakoid, increasing the H+ ions gradient.
