Right choice is (c) Depth of concrete bed block
The explanation is: The depth of concrete bed block depends upon the type of concrete, the projection of block and the soil bearing pressure. It is found on the basis of the bending moment imposed on it, and on the basis of safe modulus of rupture. Here, d = depth of concrete block in metre, a = projection beyond the masonry face in centimetres, m = safe modulus of rupture of concrete mix, q = net soil bearing pressure.