The trapping centre of light energy in photosystem-$I$ is

The source of $O_2$ liberated in photosynthesis in green plants is:

The Hill reaction occurs in:

$2 H_{2} O \rightarrow 4 H^{+} + O_{2} + 4 e^{-}$
Based on the above equation,answer the following questions:
$(a)$ Where does this reaction take place in plants?
$(b)$ What is the significance of this reaction?

Which element is present in the $OEC$ (Oxygen Evolving Complex)?

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