The correct answer is: B. 5 cm to 8 cm.
A ribbed slab is a type of concrete slab that is reinforced with steel ribs. The ribs are placed perpendicular to the direction of the load, and the concrete is poured between the ribs. The thickness of the topping of a ribbed slab is typically between 5 cm and 8 cm.
The thickness of the topping is important because it affects the strength and stiffness of the slab. A thicker topping will be stronger and stiffer than a thinner topping. However, a thicker topping will also be heavier and more expensive. The thickness of the topping should be chosen based on the loads that the slab will be subjected to and the desired strength and stiffness of the slab.
The other options are incorrect because they are not within the typical range for the thickness of the topping of a ribbed slab. Option A is too thin, and option C is too thick. Option D is not a realistic option for the thickness of the topping of a ribbed slab.