Amongst the following chemical reactions,the one representing homogeneous catalysis is

• A
  $N_{2(g)} + 3H_{2(g)} \xrightarrow{Fe(s)} 2NH_{3(g)}$
• B
  $2SO_{2(g)} + O_{2(g)} \xrightarrow{NO_{(g)}} 2SO_{3(g)}$
• C
  $CO_{(g)} + 3H_{2(g)} \xrightarrow{Ni(s)} CH_{4(g)} + H_2O_{(g)}$
• D
  $2SO_{2(g)} + O_{2(g)} \xrightarrow{V_2O_5(s)} 2SO_{3(g)}$