Identify the substrate ' S ' in the following reaction.

• A

  p-Nitrochlorobenzene
• B

  o-Nitrochlorobenzene
• C

  2,4,6 -trichloronitrobenzene
• Correct answer
  D

  2,4,6-trinitrochlorobenzene

Hence, the correct answer is option: 'D'.