Correct answer is (a) Foundation is infinitely rigid and Soil pressure is assumed to be planar
To explain I would say: The conventional method is based on the following two basic assumptions:
i) The foundation is infinitely rigid and therefore, the actual deflection of the raft does not influence the pressure distribution below the shaft.
ii) The soil pressure is assumed to be planar such that the centroid of the soil pressure coincides with the line of action of the resultant force of all the loads acting on the foundation.