c
\(\begin{array}{l}
Let\,m\,be\,mass\,of\,the\,block\,and\,L\,be\,lenght\,of\\
the\,inclined\,plane.\\
According\,to\,work - energy\,theorem\\
\,\,\,\,\,\,\,\,W = \Delta K = 0\,\,(Initial\,and\,final\,speeds\,are\,zero)\\
\therefore \,Wrok\,done\,by\,friction\, + Work\,done\,by\,gravity = 0\\
\,\,\,\,\,\,\,\,\,\, - \mu mg\cos \theta \frac{L}{2} + mg\sin \theta L = 0\\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{\mu }{2}\cos \theta  = \sin \theta \\
\,\,\,\,\,\,\,\,\,\,\,\,\mu \, = \frac{{2\sin \theta }}{{\cos \theta }} = 2\tan \,\theta 
\end{array}\)