$y=\log \left(\frac{\sqrt{x^2+1}-x}{\sqrt{x^2+1}+x}\right) \Rightarrow \frac{d y}{d x}=$

• A

  $\frac{1}{\sqrt{1+x^2}}$
• B

  $\frac{2}{\sqrt{1+x^2}}$
• C

  $\frac{-1}{\sqrt{1+x^2}}$
• Correct answer
  D

  $\frac{-2}{\sqrt{1+x^2}}$

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