$S -$ 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.

• A
  $S$ and $R$ are both true,and $R$ is the correct explanation of $S$.
• B
  $S$ and $R$ are both true,but $R$ is not the correct explanation of $S$.
• C
  $S$ is true and $R$ is false.
• D
  $S$ is false and $R$ is true.