Chromoplasts may be of which color?

In the given figure,what do '$a$' and '$b$' represent respectively?

$S -$ 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.

All plastids have similar structures because they ..........

The thylakoids in chloroplasts are arranged as

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