The decreasing order of nucleophilicity among the nucleophiles

• A

  (a), (b), (c), (d)
• B

  (d), (c), (b), (a)
• C

  (b), (c), (a), (d)
• Correct answer
  D

  (c), (b), (a), (d)

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