Figure (b) correctly depicts the forces and their relative magnitudes acting on block A while the two-block system is at rest.
Block A is at rest, so the net force acting on it must be zero which means that the forces acting on block A must balance each other out. There are three forces acting on block A:
The weight of block A (mg), acting vertically downward.
The force of friction (f), acting horizontally to the right.
The normal force (N), acting vertically upward.
The normal force (N) is equal to the weight of block A (mg) because block A is not moving vertically, so the vertical forces must be balanced.
The force of friction (f) is less than the force of static friction (µsN), where µs is the coefficient of static friction due to the fact that block A is not moving horizontally, so the force of friction is not at its maximum value.