If $\int \frac{d x}{\left(x^2+9\right) \sqrt{x^2+16}}=\frac{1}{3 \sqrt{7}} \operatorname{Tan}^{-1}\left(\mathrm{~K} \frac{x}{\sqrt{16+x^2}}\right)+\mathrm{c}$, then $\mathrm{K}=$

• Correct answer
  A

  $\frac{\sqrt{7}}{3}$
• B

  $3 \sqrt{7}$
• C

  $\frac{3}{\sqrt{7}}$
• D

  $\frac{3}{7}$

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