Figure shows the capacitor plates with dielectric placed at centre as given in question.
Electric field vectors at air and in dielectric are shown in figure. Potential is also plotted and shown in figure.
(A) As one goes from x=0 to x=3d, Magnitude of electric field remains same .
This is incorrect statement. Electric field in air space is greater than the electric field inside dielectric.
This is due to induced charges inside dielectric which are reversed in comparision with charges of capacitor plates.
(B) As one goes from x=0 to x=3d, Direction of electric field same
This is correct statement. Only magnitude of electric field is decreased inside dielectric but direction of net field is same.
(C) As one goes from x=0 to x=3d, potential increases continuously
This is incorect statement. Figure shows the potential plot as a function of distance from plate which is at x = 0.
(D) As one goes from x=0 to x=3d, potential increases at first, then decreases and again increases.
This is corect statement as seen from the potential plot as a function of distance