(b) decrease
According to the first law of limiting friction,
f = μN
where f is the frictional force
N is the normal reaction force
μ is the coefficient of static friction
and
N = mg - Fcosθ
where m is the mass of the body
F is the contact force acting on the body
If we decrease the angle between this contact force and the vertical, then Fcosθ increases and the normal reaction force (N) as well as the frictional force (f) decrease.