What is the folded membranous system of the inner membrane of the chloroplast called?

Which of the following statements are correct?
$(A)$ The number of chloroplasts varies from $1$ per cell in $Chlamydomonas$, a green alga, to $20-40$ per cell in the mesophyll.
$(B)$ Out of the two membranes of chloroplasts and mitochondria, the inner membrane is relatively less permeable.
$(C)$ Thylakoids are arranged in stacks like the piles of coins called grana (singular: granum) or the intergranal thylakoids.
$(D)$ The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins.

In the given figure,what does the part labeled '$a$' represent and what does it contain?

Choose the correct sentence.

$S -$ Statement: The flattened sac-like structures arranged like a stack of coins are called grana.
$R -$ Reason: There are $40$ to $60$ grana in a chloroplast.

Which plastid is responsible for the process of photosynthesis?