(c)
Let $u = \sin x^{3}\ and\ v = \cos x^{3}$.
On differentiating w.r.t. $x,\$we get
$\frac{du}{dx} = \cos{x^{3}.3x^{2}\ and\frac{dv}{dx} = - \sin{x^{3}.3x^{2}}}$
$\therefore\frac{du}{dv} = \frac{du/dx}{dv/dx} = \frac{3x^{2\ }\cos x^{3}}{- 3x^{2}\sin x^{3}} = - \cot x^{3}$