Let $I={\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}x\sqrt[\phantom{2}]{1+x^2}dx.$ Which substitution $is$ most effective?

$u=x$

$u=1+x^2$

$u=\sqrt[\phantom{2}]{1+x^2}$

$u=ta{n}^{-1}(x)$

$u=\frac{1}{x^2}$