(d)
Let $I = \int_{- \pi}^{\pi}{(\sin^{6}x} + \cos^{6}x)\ dx = 2\int_{0}^{\pi}{(\sin^{6}x} + \cos^{6}x)\ dx$
$= 4\int_{0}^{\frac{\pi}{2}}{\sin^{6}x}\ dx + 4\int_{0}^{\frac{\pi}{2}}{\cos^{6}\left( \frac{\pi}{2} - x \right)}\ dx$
$= 8\int_{0}^{\frac{\pi}{2}}{\sin^{6}x}\ dx = 8 \times \frac{5.3.1}{6.4.2} \times \frac{\pi}{2} = \frac{5\pi}{4}$