The correct answer is A. 6 legged.
The nominal stirrups provided in a combined footing if shear stress does not exceed 5 kg/cm2 are 6 legged. This is because the shear stress in a combined footing is usually low, and 6 legged stirrups are sufficient to provide the required shear strength.
Stirrups are used in reinforced concrete beams and slabs to resist shear forces. They are usually made of steel and are placed perpendicular to the longitudinal reinforcement. The number of legs of a stirrup refers to the number of bars that are bent at right angles to the longitudinal reinforcement.
The size and spacing of stirrups are determined by the shear stress in the concrete. The shear stress is calculated by dividing the shear force by the area of the concrete section. The shear force is the force that tries to slide one part of the concrete section over another. The area of the concrete section is the area of the cross-section of the concrete, perpendicular to the direction of the shear force.
The shear stress in a combined footing is usually low, because the footing is spread out over a large area. This means that the shear force is spread out over a large area, and the shear stress is low.
6 legged stirrups are sufficient to provide the required shear strength in a combined footing with a shear stress of 5 kg/cm2. This is because the shear strength of a stirrup is proportional to the number of legs of the stirrup. The more legs a stirrup has, the stronger it is.
In conclusion, the correct answer is A. 6 legged.