$A -$ The enzyme nitrogenase is highly sensitive to molecular oxygen.
$R -$ To protect nitrogenase enzymes,the nodule contains an oxygen scavenger called leghaemoglobin.

• A
  $A$ and $R$ are correct.
• B
  $A$ and $R$ are incorrect.
• C
  $A$ is correct,$R$ is incorrect.
• D
  $A$ is incorrect,$R$ is correct.