The enzyme required for the $CO_2$ fixation in the $C_4$ cycle is

The enzyme which catalyzes the primary carboxylation step in the photosynthetic $C_4$ cycle is:

The correlation between 'Kranz' anatomy and the $C_4$ pathway of $CO_2$ assimilation was first established by:

Given below are two statements:
Statement $I$: The primary $CO_{2}$ acceptor in $C_{4}$ plants is phosphoenolpyruvate $(PEP)$ and is found in the mesophyll cells.
Statement $II$: Mesophyll cells of $C_{4}$ plants lack $RuBisCo$ enzyme.
In the light of the above statements,choose the correct answer from the options given below:

The following figure represents the Hatch and Slack pathway. Identify $P, Q, R$ and $S$.

By looking at which internal structure of a plant can you tell whether a plant is $C_{3}$ or $C_{4}$? Explain.