Which among the following is a correct order of $\mathrm{p}^{\mathrm{kb}}$ values of amines?

• A

  $\mathrm{R}-\mathrm{NH}_2>\mathrm{NH}_3>\mathrm{R}_2 \mathrm{NH} < \mathrm{R}_3 \mathrm{~N}$
• B

  $\mathrm{R}_3 \mathrm{~N}>\mathrm{R}_2-\mathrm{NH}>\mathrm{R}-\mathrm{NH}_2>\mathrm{NH}_3$
• C

  $\mathrm{R}_2 \mathrm{NH}>\mathrm{R}_3 \mathrm{~N}>\mathrm{R}-\mathrm{NH}_2>\mathrm{NH}_3$
• Correct answer
  D

  $\mathrm{NH}_3>\mathrm{R}-\mathrm{NH}_2>\mathrm{R}_2 \mathrm{NH} < \mathrm{R}_3 \mathrm{~N}$

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