$\int \limits_0^{\pi/2} \large\frac{dx}{1+ \tan^3 x}$ is equal to :

Question

\[\begin {array} {1 1} (a)\;\pi & \quad (b)\;\frac{\pi}{2} \\ (c)\;\frac{\pi}{4} & \quad (d)\;\frac{3 \pi}{2} \end {array}\]