The correct answer is: B. Maximum bending moment
The amount of reinforcement for main bars in a slab is based on the maximum bending moment. The maximum bending moment is the maximum force that the slab will experience due to bending. The main bars are designed to resist this force.
The minimum bending moment is the minimum force that the slab will experience due to bending. The minimum bending moment is usually much smaller than the maximum bending moment, so the main bars are not designed to resist this force.
The maximum shear force is the maximum force that the slab will experience due to shear. The shear force is usually much smaller than the bending moment, so the main bars are not designed to resist this force.
The minimum shear force is the minimum force that the slab will experience due to shear. The minimum shear force is usually much smaller than the maximum shear force, so the main bars are not designed to resist this force.