Which chemical compound/molecule supplies electrons continuously to $PS-II$?

In the grana of the chloroplast,the reaction $ADP + \Pi = ATP$ during the day shows:

Which photosystem is inactive in the red drop phenomenon?

Electrons which get excited in $PS-I$ must be replaced. These replacements ultimately come from

Read the following statements:
$(a)$ $F_{0}$ part of $ATP$ase is associated with the breakdown of the proton gradient.
$(b)$ $A$ $H^+$ carrier contributes to the creation of the proton gradient.
$(c)$ Movement of electrons in $ETS$ is coupled to the pumping of protons into the lumen.
$(d)$ Formation of $NADPH + H^+$ is related to the creation of the proton gradient.
How many of the above statements are correct?

Proton gradient is very important across the membrane because