The correct answer is: B. Providing shear reinforcement.
Shear reinforcement is a type of reinforcement that is used to resist shear forces in concrete structures. It is typically made of steel bars or wires that are placed perpendicular to the direction of the shear force. Shear reinforcement can be used to increase the shear strength of concrete, and it can also help to prevent the formation of cracks.
The other options are incorrect because they will not necessarily reduce the shear stress in a slab. Increasing the depth of a slab will increase its shear strength, but it will also increase the weight of the slab. Using high strength steel will also increase the shear strength of the slab, but it will also increase the cost of the slab. Using thinner bars but more in number will not necessarily reduce the shear stress in the slab, and it may actually increase the shear stress if the bars are not properly spaced.
In conclusion, the correct answer is: B. Providing shear reinforcement.