Calcular la derivada: $\dfrac{d}{dx} \left[ \tan^{-1} \left( \dfrac{\sqrt{x}(3-x)}{1-3x} \right) \right] =$

1. [a.] $\dfrac{1}{2(1+x)\sqrt{x}}$
2. [b.] $\dfrac{3}{(1+x)\sqrt{x}}$
3. [c.] $\dfrac{2}{(1+x)\sqrt{x}}$
4. [d.] $\dfrac{3}{2(1+x)\sqrt{x}}$