The correct answer is: A. Distance between the centers of the bearings.
The effective span of a simply supported slab is the distance between the centers of the bearings. This is because the load on the slab is transferred to the supports through the bearings, and the effective span is the distance over which the load is transferred.
The other options are incorrect because they do not take into account the way that the load is transferred to the supports. Option B is the clear distance between the inner faces of the walls plus twice the thickness of the wall. However, this does not take into account the fact that the load is transferred to the supports through the bearings. Option C is the clear span plus effective depth of the slab. However, this does not take into account the fact that the load is transferred to the supports through the bearings. Option D is none of these.
The effective span of a simply supported slab is an important factor in determining the design of the slab. The effective span is used to calculate the bending moment and shear force in the slab. The bending moment and shear force are then used to determine the size and reinforcement of the slab.