$A$ certain reaction occurs in two steps as:
$(i)$ $2 SO_{2(g)} + 2 NO_{2(g)} \rightarrow 2 SO_{3(g)} + 2 NO_{(g)}$
$(ii)$ $2 NO_{(g)} + O_{2(g)} \rightarrow 2 NO_{2(g)}$
In the reaction,

• A
  $NO_{2(g)}$ is intermediate
• B
  $NO_{(g)}$ is intermediate
• C
  $NO_{(g)}$ is catalyst
• D
  $O_{2(g)}$ is intermediate