\cos ^{-1}\left(\cos \frac{8 \pi}{5}\right)= (a) \frac{8 \pi}{5} (b) \frac{12 \pi}{5} (c) \frac{2 \pi}{5} (d) \frac{4 \pi}{5}

SOL:
Correct option is D. \frac{4 \pi}{5}