$\int f(x) d x=\ln \left(x+\sqrt{x^{2}-1}\right)+c$ হলে $f(x)=$কত ?

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

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

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

$\left(x^{2}-1\right)^{\frac{3}{2}}$