Let $\mathrm{A}=[\mathrm{a}, \mathrm{b}, \mathrm{c}, \mathrm{d}], \mathrm{B}=[1,2,3]$. Relation $\mathrm{R}_1, \mathrm{R}_2, \mathrm{R}_4$ are as follows: $$ \begin{aligned} & \mathrm{R}_1=[(\mathrm{a}, 1),(\mathrm{b}, 2),(\mathrm{c}, 1),(\mathrm{d}, 2)] \\ & \mathrm{R}_2=[(\mathrm{a}, 1),(\mathrm{b}, 1),(\mathrm{c}, 1),(\mathrm{d}, 1)] \\ & \mathrm{R}_3=[(\mathrm{a}, 2),(\mathrm{b}, 3),(\mathrm{c}, 2),(\mathrm{d}, 2)] \\ & \mathrm{R}_4=[(\mathrm{a}, 1),(\mathrm{b}, 2),(\mathrm{a}, 2),(\mathrm{d}, 3)], \text { then } \end{aligned} $$

• A

  only $\mathrm{R}_3$ and $\mathrm{R}_4$ are not functions
• B

  only $R_1$ and $R_2$ are not functions
• C

  only $\mathrm{R}_3$ is not a function
• Correct answer
  D

  only $\mathrm{R}_4$ is not a function

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