Let $f(x)=sin^4x+cos^4x.$ Then f is an increasing function in the interval :

Question

$\begin{array}{1 1}(1)\; ]0, \frac{\pi}{4}[ \\ (2)\; ] \frac{\pi}{4}, \frac{\pi}{2}[ \\ (3)\; ]\frac{\pi}{2}, \frac{5 \pi}{8}[ \\ (4)\; ]\frac{5 \pi}{8} , \frac {3 \pi}{4}[\end{array} $  