Right answer is (a) E=AE0/d
To explain I would say: The field strength of the surface wave of flat earth is given by E=AE0/d, here A is the ground attenuation factor, d is the distance and E0 is the field strength per unit distance. The field strength of the wave at a unit distance from transmitting antenna depends on both on the power radiated by the transmitting antenna & directivity in vertical and horizontal planes