As Electrons Travel from the Primary Electron Acceptor in Photosystem

As electrons travel from the primary electron acceptor in photosystem II, down the electron transport chain to photosystem I, they drive the movement of protons from the stroma into the thylakoid compartment. Why is the development of this chemical gradient useful to photosynthesis?

A) The proton gradient can drive the production of ATP, which is needed for the light-independent reactions
B) Protons are directly used in the thylakoid to make glucose
C) This proton gradient is used to make NADPH, which is needed for the Calvin cycle
D) Protons provide the energy needed to drive the light-dependent reactions