In $\triangle A B C, \sqrt{\frac{r r_2}{r_3 r_1}}=$

• A

  $\left(r_3-r_2\right)\left(r_1-r_2\right)$
• B

  $r_3+r_1$
• C

  $\frac{b}{r_3-r_1}$
• Correct answer
  D

  $\frac{b}{r_3+r_1}$

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