(d) For upper half
$v^{2} = u^{2} + 2al/2 = 2\left( g\sin\theta \right)l/2 = gl\ sin\ \theta$
For lower half
$\Rightarrow 0 = u^{2} + 2g(\sin{\theta - \mu\cos\theta)\ \frac{l}{2}}$
$\Rightarrow - gl\sin\theta = gl(\sin\theta - \mu\cos\theta)$
$\Rightarrow \mu\cos\theta = 2\sin\theta \Rightarrow \mu = 2\tan\theta$