c
(c)
As plank is resting, force of friction is just equal to centripetal force due to rotation of earth.
Force trying to slip plank on the ground is parallel component of centripetal force.
$\therefore$ friction, $f=F_{||}$
$\Rightarrow f =m \omega^2 r_e \sin 45^{\circ}$
$=m \omega^2 r_e \cos 45^{\circ} \sin 45^{\circ}=\frac{m(\omega)^2 r_e}{2}$
