The correct answer is: A. Half of the width of the slab.
According to IS: 456, slabs which span in two directions with corners held down, are assumed to be divided in each direction into middle strips and edge strips such that the width of the middle strip is half of the width of the slab.
The middle strips are those strips which are located between the edge strips. The edge strips are those strips which are located at the periphery of the slab.
The width of the middle strip is calculated as follows:
Width of middle strip = Width of slab / 2
For example, if the width of the slab is 10 m, then the width of the middle strip will be 5 m.
The reason for dividing the slab into middle strips and edge strips is to simplify the analysis of the slab. The middle strips are assumed to carry the major portion of the load, while the edge strips are assumed to carry the minor portion of the load. This assumption makes the analysis of the slab much simpler.