What is the function of $RuBisCO$?

Which of the following is true for photosynthesis?

Which of the following statements is/are not true?
$I.$ In $CAM$ plants,stomata open during dark and remain closed during the day.
$II.$ Role of $Na^+$ in stomatal opening is now universally accepted.
$III.$ The water potential of root cells is higher than the water potential of soil.
$IV.$ Capillarity theory is the most accepted theory of water movement through plants.
$V.$ The walls of xylem vessels made up of ligno-cellulose have strong affinity for water molecules.

$P$: During the dark reaction of photosynthesis,ketotriose is synthesized.
$Q$: The sugar present in the structure of $ATP$ is an example of a ketopentose sugar.

Find the incorrect statement$(s)$ about photosynthesis from the following:
$A$. The water-splitting complex is associated with $PS-I$.
$B$. $C_4$ plants use the $C_3$ pathway of $CO_2$ fixation as the main biosynthetic pathway.
$C$. In $C_4$ plants, photorespiration does not occur.
$D$. $C_3$ plants exhibit 'Kranz' anatomy.
$E$. $ATP$ synthesis in chloroplast occurs through chemiosmosis.

Which of the following ions participate in the process of photosynthesis in higher plants?