A block rests on a rough plane whose inclination$\theta$ to the horizontal can be varied. Which of the following graphs indicates how the frictional force f between the block and the plane varies as $\theta$ is increased?

 Let $\alpha =$ angle of repose

For$\theta \le \alpha$ Block is stationary and force of friction, $f=mg\sin \theta$ or $f\propto \sin \theta$

i.e. it is sine graph

For $\theta \ge \alpha$ Block slides downwards

$thereforef=\mu mg\cos \theta$ or $f\propto \cos \theta$

i.e. now it is cosine graph

The correct alternative is therefore (b).