The $C_4$ and $C_3$ plants differ from each other in

Which of the following processes involves the conversion of light energy into chemical energy?

Assertion: Six molecules of $CO_2$ and twelve molecules of $NADPH + H^+$ and $18\, ATP$ are used to form one hexose molecule.
Reason: Light reaction results in the formation of $ATP$ and $NADPH_2$.

Select the correct option from the following.

$A-$ In chloroplast,the membrane system is responsible for trapping the light energy and also for the synthesis of $ATP$ and $NADPH$. This is called the light reaction.
$R-$ In the stroma,enzymatic reactions incorporate $CO_2$ into the plant,leading to the synthesis of sugar.

How many molecules of water are needed by a green plant to produce one molecule of hexose or to reduce $6$ molecules of $CO_2$?