The constant term in the expansion of $\left(1+\frac{1}{x}\right)^{20}\left(30 x(1+x)^{29}+(1+x)^{30}\right)$ is

• A

  ${ }^{50} \mathrm{C}_{20}+30 \cdot{ }^{50} \mathrm{C}_{29}$
• B

  ${ }^{50} \mathrm{C}_{19}+30 \cdot{ }^{49} \mathrm{C}_{19}$
• C

  ${ }^{50} \mathrm{C}_{20}+30 \cdot{ }^{49} \mathrm{C}_{20}$
• Correct answer
  D

  ${ }^{50} \mathrm{C}_{20}+30 \cdot{ }^{49} \mathrm{C}_{19}$

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